For me it's at ~/.mozilla/firefox-trunk/047mvdpd.default/chrome/userChrome.css (your profile id will be different, and it'll be firefox instead of firefox-trunk

For me it's at ~/.mozilla/firefox-trunk/047mvdpd.default/chrome/userChrome.css (your profile id will be different, and it'll be firefox instead of firefox-trunk if you're not running nightly).