Sometimes, the toolbar displayed on the top of the FireFox app of your Android device vanishes all of a sudden. This happened recently on my dad’s Samsung tablet and will occur if you activate full screen functionality.
It will also occur if you install any third party app which causes the screen to go on the full screen mode.
In order to fix this problem, you need to gently tap on the blank content portion after you open the FireFox app. You will view a message which prompts you to exit full screen. If you select it the toolbar reappears.
The full screen functionality has been introduced to give you more screen real estate in a size crunched phones and tablets.