Hm, it could be a new issue then. I'm not seeing it on the Bazzite issues page, perhaps you should open a new issue on it here, and provide your hardware specs. At the very least, you could see if this issue is reproducible on other installs, and someone there could help to obtain more useful debug info to determine the actual problem. You could also report it on the Mozilla Bugzilla page, as chances are this is a Firefox issue and not a Bazzite issue, but I admit that the interface for bug reports is less intuitive for non-developers there. Bazzite devs would likely direct you to there first anyway though.

All I can really say myself is that I don't experience this on Fedora Atomic KDE 40.20240607 with Mullvad Browser or the Firefox flatpak. I suspect it is either a hardware/config issue (on fresh install, I'd say a config issue is a distro issue if you haven't changed anything), or that this is Bazzite specific and not present in upstream Fedora Atomic. Regardless, it's a good idea to report this so that other users don't experience the same bug