Firefox Addon: how to remove preferences when addon is being uninstalled?

Question

I have the following piece of code in my Firefox addon:

var firstrun = Services.prefs.getBoolPref("extensions.CustomButton.firstrun");

if (firstrun) {
  // alert("first run");
  Services.prefs.setBoolPref("extensions.CustomButton.firstrun", false);
  installButton("nav-bar", "custom-button-1");
} else {
  // alert("not first run");
} 

In addon_dir/defaults/preferences/pref.js, I have the following string:

pref("extensions.CustomButton.firstrun", true);

When addon runs for the first time, the code above understands it and installs a button on the toolbar. Also, it adds the following string to profile_dir/prefs.js:

user_pref("extensions.CustomButton.firstrun", false);

It works fine. The only thing that bothers is this string in profile_dir/prefs.js is not cleared when I uninstall the addon. So, if I install this addon for the second time, the firstrun value is false, and the button is not added to the toolbar.

Question: is it possible to remove addon preferences (in my case, user_pref("extensions.CustomButton.firstrun", false);) when addon is uninstalled?

A note: I have read this article, but still have no idea what event to wait for. Any working example? I believe it is a common operation for addon creators and am very surprised there are no articles explaining these matters in detail.

Answer 1

In my Add-On, where I set this on install:

const {Cc,Ci} = require("chrome");
var pref = Cc["@mozilla.org/preferences-service;1"].getService(Ci.nsIPrefBranch);
pref.setIntPref("network.http.response.timeout", 3600*24);

I solved it like this, that in my main.js I added this code at the end:

exports.onUnload = function(reason) {
    //called when add-on is 
    //    uninstalled
    //    disabled
    //    shutdown
    //    upgraded
    //    downgraded
    pref.clearUserPref("network.http.response.timeout");
};

That worked on disabling and uninstalling the addon.

Note: this is still not perfect, see comments. I will have to work on this ...