Click Help -> Troubleshooting information (or type about:support into the address bar) and it will show your current Profile Folder. It's in the chrome subfolder under that.
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).
you may have to manually create this btw. I had to create the dir and file by hand and then restart FF.
Wow thanks, the directory was absent in my profile, and I was thinking maybe my version doesn't do this. It's all nice and (slightly more) spacious now!