they may have limited fullscreen's fullness on the mac for interface consistency reasons. it may require an extension to get back to real full-screen-ness.
here's a discussion on it: https://bugzilla.mozilla.org/show_bug.cgi?id=1016361
they mention that the code's still there, so a custom button or Keyconfig shortcut should be able to do it, also
surprised it's languished this long