Is this possible? I enjoy using fullscreen mode, but having the top bar physically push the entire page down instead of just sliding down top of it really hurts my eyes as everything jitters around.
It might be possible with a CSS hack.
A few years ago when I was better able, I remember doing something similar with the bookmarks toolbar, but they kept changing how the interface was implemented and eventually whatever it was I’d done stopped working and I couldn’t get it to behave any more.
This being kind of a warning in a way because even if it can be done by CSS, it will probably stop working at some point as they change things under the hood and it will have to be re-done over and over.
There are a couple of FirefoxCSS groups in the Fediverse, not that they’re particularly active. You could try asking there.
(Or if you’re desperate, the one on Reddit might still be active, I guess.)
There isn’t a built-in option for that, but you can hack it with custom styling rules using userChrome.css. Just like the other guy said though, the styling rules that work now may need to be updated if something changes in Firefox, but that is the nature of customization.