system>preference>screen resolution is one possible answer

Try a different resolution! some times without our desiring it a
program may change the screen resolution. try a resolution with a
larger number for the hight of the screen.

Of course I am assuming that your FF window is maximized. If the
window is not maximized try maximizing the window first (full screen).

Under preferences you will find "windows" that give preferences for
your open windows. Look and see what key you need to press and grab
the window with your mouse. Then grab the window and drag it down to
access the windows top bar.