Which Google properties are Chrome only? I'm not doubting you but the major ones (search, mail, maps, ads) are extremely cross-platform.

In the past there were features that didn't work at all; I used to hit those regularly. Device setup flows, AV features, etc. These days, it's never "this doesn't work on other browsers". It's always "this is worse on other browsers", whether because they don't test it or because they don't care.

YouTube is terrible on Firefox. There was a period where it was usable but got increasingly worse with missed frames, low frame rate. On FastMail and Gmail the expanded search overlay doesn't disappear when you click outside (ESC doesn't work), you often get stuck with it. On YouTube when you stop hovering over the "I like this" etc. on full screen video view, the tooltip doesn't disappear. It's death by a thousand cuts.

It sounds like Firefox needs to implement hardware assisted AV1? I don't see how you can plausibly blame anyone but Mozilla for this.

Firefox has had AV1 HW decode for years.

I wonder if he's using a Radeon gpu, those have had issues like video playback making the entire ui framerate drop. But it happens in steam/electron too, not only firefox. The shader cores aren't used much either, it's just a bug.