How does FF change to full screen? (Not the F11 style)

How does FF change to full screen? (Not the F11 style)-Linux general technology-Linux technology and application information. The following is a detailed description. I don't know what the key is or what it is, FF suddenly becomes full screen, but unlike pressing F11 full screen (I press F11 here and the window is full screen and only the tab bar is retained ), it only hides the outer border of the window (including the title bar). Others, such as the menu bar, toolbar, and status bar, are retained. The entire window is full of the screen. Other operations are normal. At this time, it seems that the window cannot be restored. You can only exit FF. After restarting, the window is normal. In this case, only two times have been encountered.
There is nothing to achieve this effect in the extensions currently installed and enabled.