Why would you want a quit button in the first place? The Android OS will take care for memory management anyway.

Firefox for Android does not have a Quit button because of Google's platform conventions for Android, where you're supposed to shut down an application by swiping it off the recent applications screen.

If you want a Quit button, you can go into the settings and set Firefox to erase at least one kind of data at shutdown, for example, cache. In order to implement that, Quit will now appear on the menu and you need to use Quit to trigger the data clearance.

See the "Delete data on exit" section of: Clear your browsing history and other personal data on Firefox for Android.