What version of Firefox? Graphics acceleration may be enabled in X11 but not in Firefox. Check the about:support page for status of that in the Graphics section. Last year I believe they enabled it by default in Linux due to improved driver stability, but there is still a blacklist. Chrome and Firefox both have blacklists, each with different card strings in it.

Over here it is: Compositing WebRender (I have layers.acceleration.force-enabled;true and gfx.webrender.all;true)

There's a far less common problem where you are running firefox in XRDP or ssh -YC and it is being sluggish - for that you have to force-enable xrender https://bugzilla.mozilla.org/show_bug.cgi?id=1263222 )