> With Microsoft switching to Chromium, Firefox is now the only viable

I'm not sure I understand your point. Is there a public statement from Microsoft that they are following Google on this? I don't know what Microsoft is going to do, but nothing stops them from keeping the API fully working. And if Firefox was based on Chromium too, nothing would stop Mozilla from keeping the API working either. Although I expect some forks to follow Google on this, namely other ad-tech companies with web browsers, like Yandex.