It's very possible that at some future time the Firefox team will remove some or all of the functionality that makes this possible, so enjoy it while you can.
Now that the Firefox team has removed XBL from Firefox starting with version 72, this trick no longer works. It was a good run while it lasted!
See the Bugzilla tracking page Disable MOZ_XBL in Firefox.
Extensions that allow you to run arbitrary javascript in your browser context don't work anymore with Firefox 57 and later. This is a workaround which allows you to run arbitrary javascript in your browser context, as used to be enabled by the extension userChromeJS.
It does not involve adding any extensions, and instead works only by adding (or changing) files in your Firefox user profile.
After I made this I learned of something called autoconfig.js which apparently serves a similar purpose; I haven't investigated it very much.
Place the three userChrome.* files in a /chrome directory inside your Firefox profile. If you already have a userChrome.css file, you may instead add the contents of the file here anywhere in your existing file.
Replace the contents of userChrome.js with whatever you wish; it will execute in the browser context whenever you open a new browser window. The existing contents of that script (in this repo) are a small change to the fullscreen behavior of Firefox under macOS: it restores the pre-Lion behavior (and hides the toolbar and tabs when in fullscreen mode).
To uninstall, remove the three files. If you have other content in the userChrome.css file you can remove just the part that you added during installation.
I wanted to enable pre-Lion osx fullscreen mode, and couldn't find an easy way to do it in Firefox Quantum. It's possible with unpacking, altering, and repacking files in the Firefox.app package... but that's painful and might not survive updates. Then I came upon this method, realized this trick was far more general than just altering fullscreen mode, and factored it out so that the javascript part was easily modifiable by anyone.
It relies on the fact that post-57 Firefox still allows a custom userChrome.css file, and a Firefox-specific CSS hack which can bind javascript to arbitrary DOM elements. I picked (somewhat at random) a DOM element in the browser whose existing XBL binding didn't already have a <constructor> tag, and added some JS there to load an external javascript file.
It's very possible that at some future time the Firefox team will remove some or all of the functionality that makes this possible, so enjoy it while you can.
