Detect all Firefox versions in JS

Question

This will detect any version of Firefox:

var isFirefox = navigator.userAgent.toLowerCase().indexOf('firefox') > -1;

more specifically:

if(navigator.userAgent.toLowerCase().indexOf('firefox') > -1){
     // Do Firefox-related activities
}

You may want to consider using feature-detection ala Modernizr, or a related tool, to accomplish what you need.

If you'd like to know what is the numeric version of FireFox you can use the following snippet:

var match = window.navigator.userAgent.match(/Firefox\/([0-9]+)\./);
var ver = match ? parseInt(match[1]) : 0;

For a long time I have used the alternative:

('netscape' in window) && / rv:/.test(navigator.userAgent)

because I don't trust user agent strings. Some bugs are not detectable using feature detection, so detecting the browser is required for some workarounds.

Also if you are working around a bug in Gecko, then the bug is probably also in derivatives of Firefox, and this code should work with derivatives too (Do Waterfox and Pale Moon have 'Firefox' in the user agent string?).

This script detects all versions of Firefox, for Desktop, from version 1 to 46.

It's the third time I've tried to answer this question on StackOverflow because I kept finding new ways to break my script. However, I think it's working now. It's a great exercise to learn about Firefox features and interesting to see how things have evolved. The script can be rewritten with different features, I chose ones I thought would be most useful, I would love for someone else to rewrite with other more useful features and post here, and compare results.

I placed the script in a try statement in case the user has any disabled settings in about.config. Otherwise I tested on every version of Firefox and it detects each one. I gave a brief description of what each feature is used for in the comments. I would like to do this for Webkit too but find the documentation not as good. Mozilla has easy to download previous versions and detailed releases.

// Element to display version
var outputVersion = document.getElementById("displayFoxVersion");

try {
    // Match UserAgent string with Firefox Desktop
    // Detect hybrid Gecko browsers and mobile
    if (navigator.userAgent.match(/firefox/i) &&
        !navigator.userAgent.match(/mobi|tablet|fennec|android|netscape|seamonkey|iceweasel|iceape|icecat|waterfox|gnuzilla|shadowfox|swiftfox/i)) {

        // Create Element and Array to test availability  
        var createdElement = document.createElement('div'),
            createdArray = [],
            firefoxVersion = "0";

        // Firefox 1.0 released November 9, 2004 
        // Check a current feature as being true, or NOT undefined 
        // AND check future features as EQUAL undefined
        if (typeof window.alert !== "undefined" &&
            typeof window.XPCNativeWrapper === "undefined" &&
            typeof window.URL === "undefined") {
            firefoxVersion = "1";
        }

        // Firefox 1.5 released October 15, 2003 
        // XPCNativeWrapper used to create security wrapper
        else if (typeof window.XPCNativeWrapper !== "undefined" &&
            typeof window.globalStorage === "undefined" &&
            typeof window.devicePixelRatio === "undefined" &&
            typeof createdElement.style.animation === "undefined" &&
            typeof document.querySelector === "undefined") {
            firefoxVersion = "1.5";
        }

        // Firefox 2 released October 24, 2006
        // globalStorage later deprecated in favor of localstorage
        else if (typeof window.globalStorage !== "undefined" &&
            typeof window.postMessage === "undefined") {
            firefoxVersion = "2";
        }

        // Firefox 3 released June 17, 2008
        // postMessage for cross window messaging
        else if (typeof window.postMessage !== "undefined" &&
            typeof document.querySelector === "undefined") {
            firefoxVersion = "3";
        }

        // Firefox 3.5 released June 30, 2009
        // querySelector returns list of the elements from document
        else if (typeof document.querySelector !== "undefined" &&
            typeof window.mozRequestAnimationFrame === "undefined" &&
            typeof Reflect === "undefined") {
            firefoxVersion = "3.5";
        }

        // Firefox 4 released March 22, 2011
        // window.URL is Gecko, Webkit is window.webkitURL, manages object URLs
        else if (typeof window.URL !== "undefined" &&
            typeof createdElement.style.MozAnimation === "undefined") {
            firefoxVersion = "4";
        }

        // After April 2011 releases every six weeks on Tuesday

        // Firefox 5 released June 21, 2011
        // style.MozAnimation for CSS animation, renamed to style.animation
        else if (typeof createdElement.style.MozAnimation !== "undefined" &&
            typeof WeakMap === "undefined") {
            firefoxVersion = "5";
        }

        // Firefox 6 released August 16, 2011
        // WeakMap collects key value pairs weakly referenced
        else if (typeof WeakMap !== "undefined" &&
            typeof createdElement.style.textOverflow === "undefined") {
            firefoxVersion = "6";
        }

        // Firefox 7 released September 27, 2011
        // textOverflow manages overflowed non displayed content
        else if (typeof createdElement.style.textOverflow !== "undefined" &&
            typeof createdElement.insertAdjacentHTML === "undefined") {
            firefoxVersion = "7";
        }

        // Firefox 8 released November 8, 2011
        // insertAdjacentHTML parses as HTML and inserts into specified position
        // faster than direct innerHTML manipulation and
        // appends without affecting other elements under the same parent
        else if (typeof createdElement.insertAdjacentHTML !== "undefined" &&
            typeof navigator.doNotTrack === "undefined") {
            firefoxVersion = "8";
        }

        // Firefox 9 released December 20, 2011
        // mozIndexedDB dropped ver 16, renamed window.indexedDB 
        // IndexDB improved functionality than localstorage
        else if (typeof window.mozIndexedDB !== "undefined" &&
            typeof document.mozFullScreenEnabled === "undefined") {
            firefoxVersion = "9";
        }

        // Firefox 10 released January 31, 2012
        // mozFullScreenEnabled reports if full-screen mode is available
        else if (typeof document.mozFullScreenEnabled !== "undefined" &&
            typeof window.mozCancelAnimationFrame === "undefined" &&
            typeof Reflect === "undefined") {
            firefoxVersion = "10";
        }

        // Firefox 11 released March 13, 2012
        // mozCancelAnimationFrame prior to Firefox 23 prefixed with moz
        // Cancels an animation frame request
        else if (typeof window.mozCancelAnimationFrame !== "undefined" &&
            typeof createdElement.style.MozTextAlignLast === "undefined") {
            firefoxVersion = "11";
        }

        // Firefox 12 released April 24, 2012
        // MozTextAlignLast how the last line is aligned
        else if (typeof createdElement.style.MozTextAlignLast !== "undefined" &&
            typeof createdElement.style.MozOpacity !== "undefined") {
            firefoxVersion = "12";
        }

        // Firefox 13 released June 5, 2012
        // MozOpacity dropped from this version
        else if (typeof createdElement.style.MozOpacity === "undefined" &&
            typeof window.globalStorage !== "undefined") {
            firefoxVersion = "13";
        }

        // Firefox 14 released June 26, 2012
        // globalStorage dropped from this version
        else if (typeof window.globalStorage === "undefined" &&
            typeof createdElement.style.borderImage === "undefined" &&
            typeof document.querySelector !== "undefined") {
            firefoxVersion = "14";
        }

        // Firefox 15 released August 28, 2012
        // borderImage allows drawing an image on the borders of elements
        else if (typeof createdElement.style.borderImage !== "undefined" &&
            typeof createdElement.style.animation === "undefined") {
            firefoxVersion = "15";
        }

        // Firefox 16 released October 9, 2012
        // animation was MozAnimation
        else if (typeof createdElement.style.animation !== "undefined" &&
            typeof createdElement.style.iterator === "undefined" &&
            typeof Math.hypot === "undefined") {
            firefoxVersion = "16";
        }

        // Firefox 17 released November 20, 2012
        // version 27 drops iterator and renames italic
        // Used to iterate over enumerable properties of an object
        else if (typeof createdElement.style.iterator !== "undefined" &&
            typeof window.devicePixelRatio === "undefined") {
            firefoxVersion = "17";
        }

        // Firefox 18 released January 8, 2013
        // devicePixelRatio returns ratio of one vertical pixel between devices
        else if (typeof window.devicePixelRatio !== "undefined" &&
            typeof window.getInterface === "undefined" &&
            typeof createdElement.style.mixBlendMode === "undefined") {
            firefoxVersion = "18";
        }

        // Firefox 19 released February 19, 2013
        // getInterface dropped and renamed in version 32
        // Retrieves specified interface pointers
        else if (typeof window.getInterface !== "undefined" &&
            typeof Math.imul === "undefined") {
            firefoxVersion = "19";
        }

        // Firefox 20 released April 2, 2013
        // Math.imul provides fast 32 bit integer multiplication
        else if (typeof Math.imul !== "undefined" &&
            typeof window.crypto.getRandomValues === "undefined") {
            firefoxVersion = "20";
        }

        // Firefox 21 released May 14, 2013
        // getRandomValues lets you get cryptographically random values
        else if (typeof window.crypto.getRandomValues !== "undefined" &&
            typeof createdElement.style.flex === "undefined") {
            firefoxVersion = "21";
        }

        // Firefox 22 released June 25, 2013
        // flex can alter dimensions to fill available space
        else if (typeof createdElement.style.flex !== "undefined" &&
            typeof window.cancelAnimationFrame === "undefined") {
            firefoxVersion = "22";
        }

        // Firefox 23 released August 6, 2013
        // cancelAnimationFrame was mozCancelAnimationFrame
        else if (typeof window.cancelAnimationFrame !== "undefined" &&
            typeof document.loadBindingDocument !== "undefined" &&
            typeof Math.trunc === "undefined") {
            firefoxVersion = "23";
        }

        // Firefox 24 released September 17, 2013
        // loadBindingDocument dropped
        // loadBindingDocument reintroduced in 25 then dropped again in 26 
        else if (typeof document.loadBindingDocument === "undefined" &&
            typeof Math.trunc === "undefined") {
            firefoxVersion = "24";
        }

        // Firefox 25 released October 29, 2013
        // Math.trunc returns number removing fractional digits
        else if (typeof Math.trunc !== "undefined" &&
            typeof document.loadBindingDocument !== "undefined") {
            firefoxVersion = "25";
        }

        // Firefox 26 released December 10, 2013
        // loadBindingDocument dropped
        else if (typeof Math.trunc !== "undefined" &&
            typeof Math.hypot === "undefined") {
            firefoxVersion = "26";
        }

        // Firefox 27 released February 4, 2014
        // Math.hypot returns square root of the sum of squares
        else if (typeof Math.hypot !== "undefined" &&
            typeof createdArray.entries === "undefined") {
            firefoxVersion = "27";
        }

        // Firefox 28 released March 18, 2014
        // entries returns key value pairs for arrays
        else if (typeof createdArray.entries !== "undefined" &&
            typeof createdElement.style.boxSizing === "undefined") {
            firefoxVersion = "28";
        }

        // Firefox 29 released April 29, 2014
        // boxSizing alters CSS box model, calculates width and height of elements
        else if (typeof createdElement.style.boxSizing != "undefined" &&
            typeof createdElement.style.backgroundBlendMode === "undefined") {
            firefoxVersion = "29";
        }

        // Firefox 30 released June 10, 2014
        // backgroundBlendMode blends elements background images
        else if (typeof createdElement.style.backgroundBlendMode !== "undefined" &&
            typeof createdElement.style.paintOrder === "undefined") {
            firefoxVersion = "30";
        }

        // Firefox 31 released July 22, 2014
        // paintOrder specifies the order fill, stroke, markers of shape or element
        else if (typeof createdElement.style.paintOrder !== "undefined" &&
            typeof createdElement.style.mixBlendMode === "undefined") {
            firefoxVersion = "31";
        }

        // Firefox 32 released September 2, 2014
        // mixBlendMode how an element should blend 
        else if (typeof createdElement.style.mixBlendMode !== "undefined" &&
            typeof Number.toInteger !== "undefined") {
            firefoxVersion = "32";
        }

        // Firefox 33 released October 14, 2014
        // numberToIntger dropped, used to convert values to integer
        else if (typeof Number.toInteger === "undefined" &&
            typeof createdElement.style.fontFeatureSettings === "undefined") {
            firefoxVersion = "33";
        }

        // Firefox 34 released December 1, 2014
        // fontFeatureSettings control over advanced typographic features
        else if (typeof createdElement.style.fontFeatureSettings !== "undefined" &&
            typeof navigator.mozIsLocallyAvailable !== "undefined") {
            firefoxVersion = "34";
        }

        // Firefox 35 released January 13, 2015
        // mozIsLocallyAvailable dropped
        else if (typeof navigator.mozIsLocallyAvailable === "undefined" &&
            typeof createdElement.style.MozWindowDragging === "undefined") {
            firefoxVersion = "35";
        }

        // Firefox 36 released February 24, 2015
        // quote returns a copy of the string
        else if (typeof String.quote !== "undefined" &&
            typeof createdElement.style.MozWindowDragging !== "undefined") {
            firefoxVersion = "36";
        }

        // Firefox 37 released March 31, 2015
        // quote quickly dropped
        else if (typeof String.quote === "undefined" &&
            typeof createdElement.style.rubyPosition === "undefined") {
            firefoxVersion = "37";
        }

        // Firefox 38 released May 12, 2015
        // rubyPosition defines position of a ruby element relative to its base element
        else if (typeof createdElement.style.rubyPosition !== "undefined" &&
            typeof window.Headers === "undefined") {
            firefoxVersion = "38";
        }

        // Firefox 39 released July 2, 2015
        // Headers allows us to create our own headers objects 
        else if (typeof window.Headers !== "undefined" &&
            typeof Symbol.match === "undefined") {
            firefoxVersion = "39";
        }

        // Firefox 40 released August 11, 2015
        // match matches a regular expression against a string
        else if (typeof Symbol.match !== "undefined" &&
            typeof Symbol.species === "undefined") {
            firefoxVersion = "40";
        }

        // Firefox 41 released September 22, 2015
        // species allows subclasses to over ride the default constructor
        else if (typeof Symbol.species !== "undefined" &&
            typeof Reflect === "undefined") {
            firefoxVersion = "41";
        }

        // Firefox 42 released November 3, 2015
        // mozRequestAnimationFrame and mozFullScreenEnabled dropped
        // Reflect offers methods for interceptable JavaScript operations
        else if (typeof Reflect !== "undefined" &&
            typeof window.screen.orientation === "undefined") {
            firefoxVersion = "42";
        }

        // Firefox 43 released December 15, 2015
        // orientation is mozOrientation in B2G and Android
        else if (typeof window.screen.orientation !== "undefined" &&
            typeof document.charset === "undefined") {
            firefoxVersion = "43";
        }

        // Firefox 44 released January 26, 2016
        // charset is for legacy, use document.characterSet
        else if (typeof document.charset !== "undefined" &&
            typeof window.onstorage === "undefined") {
            firefoxVersion = "44";
        }

        // Firefox 45 released March 8, 2016
        // onstorage contains an event handler that runs when the storage event fires
        else if (typeof window.onstorage !== "undefined" &&
            typeof window.onabsolutedeviceorientation === "undefined") {
            firefoxVersion = "45";
        }

        // Firefox 46 - beta
        // onabsolutedeviceorientation
        else if (typeof window.onabsolutedeviceorientation !== "undefined") {
            firefoxVersion = "46 or above";
        }

        // Else could not verify
        else {
            outputVersion.innerHTML = "Could not verify Mozilla Firefox";
        }

        // Display Firefox version
        outputVersion.innerHTML = "Verified as Mozilla Firefox " + firefoxVersion;

        // Else not detected
    } else {
        outputVersion.innerHTML = "Mozilla Firefox not detected";
    }
} catch (e) {
    // Statement to handle exceptions
    outputVersion.innerHTML = "An error occured. This could be because the default settings in Firefox have changed. Check about.config ";
}

<div id="displayFoxVersion"></div>