They could, except that would effectively show Google that it can dictate what other browsers implement, alter its products on rapid basis to regularly break them etc. and at that point there's almost no point to an alternative, since we'd we're fully back in the IE era again, which is why I don't think it's a good idea.
If one cannot avoid it, I think it's a better idea to create Chrome desktop shortcuts for Gmail/YouTube and use Chrome exclusively for that, if you cannot use a desktop email client for Gmail and VLC/mpv/youtube-dl for some reason.
Firefox doesn't have the market share to push back on Google. The best way for Firefox to grow is to make the best browser from the user's perspective.
I get that, but this seems like controlled opposition.
Moreover, once you give in on this, what's Google going to do next? Use APIs only in Chrome that Mozilla needs to implement only after they're made public in Chrome by literally looking at the source code? There's always going to be a lag if that's the dynamic, so there's always going to be the perception that Firefox is behind.
Moreover if Firefox adopts it, it makes it more likely to be adopted by Apple too, since Google's now not the only kid on the block to support it and now you turned it into a de-facto standard.
Mozilla already partially caved to Google in pursuit of the "best browser" as perceived by the average user. That was on DRM. Now I say partially because at least they made it opt-in, but so they caved and next Google came up with this thing.
If you going to keep paying ransom, you're going to have a lot of hostages.
If one cannot avoid it, I think it's a better idea to create Chrome desktop shortcuts for Gmail/YouTube and use Chrome exclusively for that, if you cannot use a desktop email client for Gmail and VLC/mpv/youtube-dl for some reason.