Google's proposed "Web Integrity API" browser-DRM was probably the biggest attack on the open web since its conception. I don't think they have fully given up on that idea and they'll likely sneak it in more gradually and slowly. Manifest v3 is just a small baby step in this direction of taking away user control.
The various websites will just say Firefox is "not supported". I just wrote this in another comment, but Twitch doesn't let you log in on FF because it has some kind of advanced tracking protection. I guess YouTube and the rest will just join the fuckery and block you from using their content if you're on FF. I mean, I really hope they won't do that, but knowing what degree of assholery these companies can pull off, I think it's the next step.
Presumably the FTC will have something to say about blatant anticompetitive actions such at these. Then again, that's why corporations buy themselves representatives, senators and judges.