Try starting Firefox in Safe mode and resetting the toolbars and controls.

Menu Bar (File, Edit, View, History, Bookmarks, Tools, Help)

Press ALT+ V T M on your keyboard. This should restore the menu bar (Firefox 3.6)

Other toolbars

View|Toolbars|Select the desired Toolbar

Other steps to try

If you missing specific icons (ex. The back arrow,) try restoring the default set by going to View|Toolbars|Customize| Restore Default Set

Safe Mode

You May need to reset toolbars and controls via Diagnose Firefox issues using Troubleshoot Mode Make sure you completely close Firefox first. When you get to the Safe Mode window, select Reset toolbars and controls and then Make Changes and Restart.

Hope this helps!