Disabling the mouseover to reveal the address/toolbar while in fullscreen - old method doesn't work

For a long time I used a known method to disable the mouseover to reveal the address/toolbar while in fullscreen. It is described here [https://support.mozilla.org/en-US/questions/1271399 link text] and works via adding this line #fullscr-toggler { display:none !important; } in userChrome.css and enabling userChrome.css at about:config page. However it stopped working for me. I am currently with Firefox 85.0.