Do you mean you can see your desktop through the toolbar area? You can replace your current Firefox theme with a different one and see whether it is "solid." More info on themes: [[Use themes to change the look of Firefox]] If you want the dark navy background from Firefox 57-88, someone created an add-on theme that emulates it. You can give it a try here: https://addons.mozilla.org/firefox/addon/photon-colors/ <img src="https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2021-06-02-17-19-21-6cf38c.png"> If pre-made themes don't suit you, the Firefox Color extension lets you create your own theme: https://addons.mozilla.org/firefox/addon/firefox-color/

Maybe switch to a non Aero Windows theme.

Yeah, I just changed windows themes and now the top of Firefox is transparent. see screenshot