What’s an “App Tab”, you ask? Well, it just so happens to be a new feature in version 4 of Mozilla Firefox! What it does is allow you to pin tabs you use frequently so they take up less space.
Just right-click a tab and choose Pin as App Tab.
This feature is handy for those that like to keep a web mail site open, but have a billion other places they need to be virtually – like me!
If you want to unpin, simply right-click the App Tab you created and choose Unpin Tab.