Firefox userChrome.css
i googled "cool words" for that title, by the way
Find your Firefox profile folder, create (if it's not already there) a "chrome" folder, and place userChrome.css in there. Quick restart, and you're done.
rip stylish
Font used is Lemon.
If you want the new tab page, place the userContent.css file in the chrome folder too.
Also, I originally had some css that was a kind of bright, all-in-one-line kinda deal from somebody on /r/FirefoxCSS, so shoutout to that guy for doing like half the work. I just moved some stuff and changed the colours.