The default Firefox 3 Mac theme does not show favicons for bookmarks you’ve added to your Bookmarks Toolbar. This is probably a decision that comes with mimicking the Safari theme which doesn’t display favicons either. Put simply, it sucks!
The Smart Bookmarks Bar extension which hides the names of bookmarks showing only the icon is currently not compatible with Firefox 3 Beta 5. It runs with the help of Nightly Tester Tools extension, but it is not very good at hiding the names of bookmarks. Even though it restores favicons on the Bookmark Toolbar without the use of the Stylish style mentioned above, it doesn’t work when it comes to hiding the names of bookmarks. You would have to go in to Organize Bookmarks and delete the names of bookmarks manually. You could just install the Stylish extension and delete the names manually, but you have control over the spacing between the icons with Smart Bookmarks bar.