Sarah asked me yesterday to help her with editing Firefox’s userChrome.css to show favicons in the Bookmarks Toolbar. Vlad had written up a post about it, but many commenters had tweaks and fixes.
So I’ve put all the code I used together onto GitHub here: http://gist.github.com/186269. Add that to your ~/Library/Application Support/Firefox/profiles/*.default/chrome/userChrome.css (or create it if it’s not there). And you should be ‘good to go’!