Make sure Firefox isn't set to run in Full Screen mode (press F11 or Fn+F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/en-US/kb/how-make-firefox-and-websites-go-full-screen

When you are in Full Screen mode, hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear. Right-click empty space on a toolbar to find "Exit Full Screen mode" and "Hide toolbars".

You can use one of these to leave Full Screen mode:

• press the F11 key
• click the Maximize button at the right end of the Tab bar
• click the Full Screen button in the "3-bar" Firefox menu button drop-down list

You can try to open the system menu via "Alt+Space" and resize the window in case the toolbars are out of view.