Could turn out to be a good thing. All power users will dump Chrome practically overnight, a huge boon to the alternatives, that could actually give them enough momentum to compete with Google for a change. I'm sure they've considered this, probably an empty treat.
I'm not sure how wide the intersection of power users that use uBO but also haven't heard of the manifest v3 deprecation coming since like 2019 actually is, but that could be because I'm the type of person to randomly recommend browsers to people and discuss them a lot.
Every browser is either chromium (open source captured by Google) or exists because of a Google search contract (this represents 80% of Mozilla's revenue), Google can't lose