When did this start?

Could you double-check to make sure Firefox is not in its full screen mode? The F11 function key switches between regular and full screen views, or you can slide the mouse pointer to the top of the screen and watch for the toolbars to slide down, the right-click a blank area near the top and choose Exit Full Screen for the short context menu. Next to your post (Question Details > More System Details) you shared your extensions list, thank you. That includes a "kiosk" extension which may hide parts of the UI not intended to be available on a public-facing browser. Could you try disabling that? See whether you can use the Add-ons page using the keyboard shortcut Ctrl+Shift+a, and then in the left column, click Extensions. If that doesn't help, try Firefox's Safe Mode. In Safe Mode, Firefox temporarily deactivates extensions and some other advanced features to help you assess whether these are causing the problem. ''If Firefox is not running:'' Hold down the Shift key when starting Firefox. ''If Firefox is running:'' You can restart Firefox in Safe Mode using either: * "3-bar" menu button > "?" button > Restart with Add-ons Disabled * Help menu > Restart with Add-ons Disabled and OK the restart. ''Both scenarios:'' A small dialog should appear. Click "Start in Safe Mode" (''not'' Refresh). In Safe Mode, you can remove extensions and possibly access their settings on the Add-ons page.