For Android version 4 and above, quit was removed from the Firefox menu to meet Google's application guidelines.

You can still close Firefox or any other app from the app switch screen, by using the app swiching button . (If you do not see the above button long press on the home button to bring up app switch screeen)

If you wish, you can re-add quit. Here's how.

How to add Quit back using a Firefox for Android add-on

Here's how to install an add-on to bring back Quit so you can close Firefox.

1. Tap the Menu button (either at the bottom right on some devices or in the upper right of the browser) , then tap Tools and tap Add-ons.
2. Tap the orange Folder icon in the upper-right of the Add-ons screen. Alternatively you could go directly to https://addons.mozilla.org/android/ to achieve the same result.
   
   
3. Search for quitnow
   
   
   
   and tap on it.
   
   
   
   
4. Tap Add to Firefox
   
   
   
   and then tap Install to install the QuitNow add-on. Your Firefox menu will now have a Quit button that allows you to exit Firefox for Android.