Steps:

1. Launch firefox and open the hamburger menu.
2. Click on the full screen button.
3. Open the hamburger menu and hover the mouse cursor over the full screen button while the browser is in full screen mode.

Actual result:

The full screen button tooltip says "Display the window in full screen" even if the user is already in full screen mode and the button icon suggest close/exit from the full screen.

Expected result:

Since the button icon is correctly updated, the button tooltip should be also updated accordingly. The full screen button tooltip should display something like "Exit/Close the full screen mode" or "Display the window in normal/default screen".