Hi  ! Would you please try this : Type in the address bar '''about:config''' (press Enter) (promise to be careful, if asked) Type and look for the preference : '''browser.fullscreen.autohide''' and set its value to '''true'''

It's already set to true

Let's try Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem. If Firefox is not running : Hold down the Shift key when starting Firefox. If Firefox is running : 3-bar menu => ' ? ' (at the bottom of the dropdown) => 'Restart with Add-ons disabled'. Then OK the restart. A small dialog should appear. Click 'Start in Safe Mode' ( not 'Refresh' !) See : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode And also : https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems --------------------------------------------------------------------------------------------------------- If that doesn't do the trick, would you take a look at this article : [https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save How to fix preferences that won't save]

It was the theme I've been using for over a year, Firefox 55 broke it...this is frustrating me to epic proportions.