The Quit option is available only when you're willing to clear some of your browsing data on exit. It's for instance cache, tabs, history etc.

1. Tap on the menu button, then Settings , followed by Privacy.
2. Put a check mark next to Always clear when quitting and select at least one type of data to clear.
3. The Quit option will show up in the menu.

On Android 4 and higher, you can close Firefox or any other app from the app switch screen.

• Ice Cream Sandwich: long press on the home button.
• Jelly Bean: tap on the app switch button .

The check mark won't allow me to check it!! Even though I : "Put a check mark next to Always clear when quitting and select at least one type of data to clear." it will not allow me to check the box after doing the above. It won't allow me to "quit" this browser is becoming useless