Fullscreen Firefox on macOS

Taken from this answer on StackExchange.

I'm on MacOS, January 2022. To accomplish what we want:

  1. Go to Firefox "full screen" mode as normal (e.g. Ctrl-Cmd-F)
  2. On upper right of the screen is the "hamburger menu" (three horizontal lines); right-click it, then select "Hide Toolbars".

No Firefox extension needed.