I see literally no incentive for them to limit their API to just the Google version of Manifest v3. If I recall correctly they already expanded their implementation to offer more.

If they ever implement WEI (the "web DRM") then it's because the Internet forces their hand. A web browser is only as useful as the websites it can browse. If our banks will demand WEI support Mozilla doesn't have much choice. We need to worry about the website providers changing the web, not Mozilla adjusting Firefox to these changes. In the latter case it's already too late and it's hard to blame Mozilla at this point.