There are posts on the net on how to fix this, none that worked for me!
(I'm sure there is a workaround using terminal commands!)
For usability, just press the F11 key on your keyboard, (you may need
to press it twice as it toggles full screen, the faulty view, and the
one that exposes the required buttons. You need to do this each time
you open up firefox. I believe that if you install the non-ubuntu
version of firefox you don't get this problem, although I haven't
tried this myself.