On the View menu, I clicked "Full screen View", but then I couldn't find any way back to normal view. I had to Force-Quit Firefox to escape.
How do you exit from Full Screen View?
(I tried Esc, Ctrl-C, and many other key combinations)
I found the answer: Use shift-flower-F, the same was used to enter Full Screen Mode
F11 "pushes" Firefox to the sides of the screen.