Some Firefox installations, such as in Ubuntu, automatically display the system title bar. It is redundant with the tab bar title and wastes precious screen real estate.
The system's title bar can easily be hidden for Mozilla Firefox from its Customize panel.
Step-by-step video guide:
Steps to hide system title bar for Mozilla Firefox:
Open
Firefox menu by clicking on the hamburger menu icon on the top right of the window.

Click on the
Customize menu.

Uncheck the
Title Bar checkbox on the bottom left of the window.

System title bar should no longer be displayed.

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