Firefox remains very conservative on enabling modern features on X11. Some distributions force them on, but otherwise it's up to the user to figure out how to do that.
It's likely that some hwaccel flag in about:config wasn't turned on by default. Similarly, if you want smooth touchpad scrolling, you need to set MOZ_USE_XINPUT2
My main Firefox in that setup is from the Mozilla repos, rather than the ESR version that is the default in Debian stable. So, it could very well be that. I will have to check to see what the ESR Firefox from the Debian repos does.
> Firefox remains very conservative on enabling modern features on X11.
So old school throthling if you don't use the "right" version (Apple batterygate, Microsoft wordperfectgate). They could blame it on testing though (we only use Wayland and we are too lazy to test the X11 version)
It's likely that some hwaccel flag in about:config wasn't turned on by default. Similarly, if you want smooth touchpad scrolling, you need to set MOZ_USE_XINPUT2