Firefox

Unfortunately you're probably right. Vivaldi has already said they will likely adopt this standard despite them disagreeing with it, I assume the same will happen to Firefox and Brave if the standard becomes widely adopted and used enough. Its not an easy issue to tackle. The good thing is we can fight back and push its adoption back as far as possible, as well as just avoiding and boycotting any websites that adopt the standard. I don't know if the push back will be big enough to make an impact, but we at least have to try and do what we can.

We've already seen DRM garbage added to nearly every browser for media playback, despite massive backlash and concerns from organizations like the EFF. Mozilla didn't want to adopt it iirc but they caved in to not lose market share and adopted it in the most user friendly and secure/privacy respective way that they could (Restricting the DRM in its own sandbox), so I could see something like that happen again unfortunately. However to be fair, this new Google DRM standard will be significantly worse and more of a problem than that DRM implementation, as this effects entire websites themselves now and is on a whole new scale and precedent, and not just for certain media content, so hopefully more can be done to prevent this and fight back.