Like seeekerman who initiated the now-closed discussion at: https://support.mozilla.org/en-US/questions/1433369, I have used Firefox since its first release. With one exception - open image in new tab - I never use tabs. I want all the usable vertical space I can get. For some time, Firefox on Windows (Windows 11, FF 143.0.4) has been showing a tab bar under the menu items, and above the bar with the URL etc. How can I hide this when the FF window has only one tab, which is almost all the time for me? The solution provided to seeekerman in that discussion, on 2023-12-08, was to create a userChrome.css file with some special incantations. I tried this it did not work.