Since I recommend Firefox all the time, I thought I’d put a note about this. There are solutions in this Ubuntu forum thread, but for those who might not get there because they’re thinking of the problem as one with opening a new tab, let me note the solution that worked for me, which is in the post of March 16, 2009 by user lkoniecki.
1. Unmaximize your Firefox. (I usually work with Firefox maximized.)
2. Size the window until it fits fully on the panel. In order to get to where I could resize the window, I had to disable all my toolbars.
3. Close Firefox so it saves the window size.
4. Reopen and enjoy. Or so it worked for me. There were several other solutions listed, but I tried this one first and it worked. If it didn’t, keep looking!