Firefox for Android will add Quit to the menu if you enable clearing of personal data at shutdown. You don't have to clear important data like history or cookies, there are several categories to choose from. You can set that up using:

menu > Settings > Privacy

At the bottom, tap Clear on Exit and select a category you don't use (e.g., saved passwords) or don't care to keep (e.g., cache) and then tap Set.

Or if you don't want to use that method, you can install this add-on: https://addons.mozilla.org/android/addon/quitnow/

By the way, Firefox is designed in accordance with Google's platform guidelines for newer versions of Android. Users can tap the icon to the right of the device home icon to call up a list of running applications, and then swipe an application off the list to close it.