I got screencaps in Thunar (XFCE) and Konqueror (KDE). I don't have Nautilus nor whatever the new file manager for KDE4's called, though.

And I just thought that for Linux, using the terminal would be easier, so here are the instructions, for you to copy-paste where you need them:

$ cd ~/mozilla/firefox/
$ ls

At this point, there should be a couple of files as well as at least one folder, whose name is a random string of letters and digits, plus ".default". This is your profile folder.

$ cd ########.default/chrome/
$ ls

Now you are in the Chrome folder, which should contain two files called userChrome-example.css and userContent-example.css. Create a user stylesheet by copying the existing example and open it in your favourite text-editor:

$ cp userContent-example.css userContent.css
$ nano userContent.css