This userChrome.css mimics the elementary GTK+ theme to make firefox blend in better in elementary OS.
I don't understand Firefox styling well enough to be able to support both window control styles, which is why this hasn't been merged back up stream. Install the two versions as separate themes.
- Go to about:support in Firefox
- Application Basics > Profile Directory > Open Directory
- Create a folder named
chrome
- Paste userChrome.css in this folder