here is the video: https://streamable.com/zffknq the issue is about transparent bar with vertical addons and slowly fullscreen animation, and the fullscreen animation isn't config-able, I already set full-screen-api.transition-duration.enter 0 0 full-screen-api.transition-duration.leave 0 0 in about:config, and disable sidebar or userChrome is not work too. and this is happend about one mouth ago after my nixos update firefox.

           #navigator-toolbox[fullscreenShouldAnimate] {
               transition: none !important;
           }