There are lots of ways I could see it happening. Firefox is still heavy on resources on low end systems. GPU is heavy, it has poor hwdec support on things like v4l2 last time I tried it (though they do at least support it now). They don’t push the envelope in any way. Firefox STILL doesn’t have JXL support despite safari supporting it (various forks of firefox support it thanks to patches firefox refuses to look at). HEVC support when platform support is available would be nice too. And these are just the issues off the top of my head.
What to prioritize? all of it. They have enough resources to do so.