As far as I know, for some reason the developers thought it was a great idea to not have a quit button. So the only way you can close Firefox for Android is by force closing it with your recent apps menu. There used to be an extension that added a quit button but that stopped working a few versions ago.

The quit button is built into ff android now! It's in settings clear data on exit. I wonder if it's just not shown when the is no data to clear

Oh so it is! Pity that it only seems to only be a 'privacy' option but I guess selecting to clear open tabs is good enough if you want nothing else cleared. It shows up for me, even after using it and having no tabs open. You came here to ask a question and ended up helping me with something instead lol, thanks. Have you tried deselecting all the options to remove the quit button, then reselecting the options you want to see if it shows up again?

Yea I have. It just seems random and I think it maybe has something to do with maybe certain web pages