Hi wimom,

I'd try starting Firefox in Diagnose Firefox issues using Troubleshoot Mode. If you don't have the issue while all of your add-ons, extensions, and themes are disabled, you can try adding them back in one by one until you find the culprit. You should look at the Extensions and Themes troubleshooting guide and the Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems article as well.

Hopefully this helps!

Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: command+Shift+F).
If you are in full screen mode then hover the mouse to the top to make the Navigation Toolbar and Tab bar appear.
You can click the Maximize button at the top right to leave full screen mode or right click empty space on a toolbar and use "Exit Full Screen Mode" or press F11.

If the menu bar is hidden then press the F10 key or hold down the Alt key to make the menu bar appear.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible: "View > Toolbars"