I just upgraded my installation of Firefox 3 to the latest Beta 5 version, after having used Beta 4 for a while. I must say that Firefox 3 on the Mac is miles above version 2, especially with respect to speed and memory usage, and it has immediately become my browser of choice, despite the unavailability of most add-ons for this version.
Beta 5, however, has this very annoying bug, at least on my Tiger-powered MacBook Pro: The Back and Forward navigation buttons are duplicated in the toolbar. Urgh!
Update: customizing the toolbar using the appropriate dialog box (View > Toolbars > Customize) to remove the extra navigation control fixed it.
https://bugzilla.mozilla.org/show_bug.cgi?id=425079
Great that beta testing exists, so your annoyance helps lessen mine.