Firefox addon to execute Javascript on specified pages?

I'd like to find a Firefox addon that would look at the url you're viewing, match it against a pattern, and if there was a match it would execute arbitrary Javascript (maybe by loading the .js file from another url).

I can't seem to find any such thing, but it must exist. Anyone seen something like this?

What is executeScript ()?

The executeScript command executes a snippet of JavaScript in the context of the currently selected frame or window. The script fragment will be executed as the body of an anonymous function. To store the return value, use the 'return' keyword and provide a variable name in the value input field.

Why is my JavaScript not working on Firefox?

From the Tools menu, click Options (on PC) or Preferences (on Mac). Click the Content tab. The Content tab is a hub of options that affect the fonts, languages, and media that your Firefox browser can display. Click Enable JavaScript or Enable Java .

Check out GreaseMonkey:

https://addons.mozilla.org/en-US/firefox/addon/748/