Some people like to leave their running apps open, while others like to close them. Appropriately named, Quit Fennec, lets you close Firefox for Android when you press the “close” button on the last tab you have open.
The Android OS automatically pauses applications when they are in the background, and closes them when free memory is needed. So most users shouldn’t need to “quit” applications in Android. But for developers, testers, and add-on authors, it is useful to have a quick way to quit and restart Firefox for Android. Enjoy!