1. It's a standard discussion point, not my argument in particular. It's the same one as for why it's a problem to have so much corporate money behind news media, political campaigns, and just about anything else.
   But
2. It's all speculation, both the idea of priority manipulation happening and your idea it does not. The general population doesn't know anyone at these projects, so everything has to be discussed in vague generalities. You can say 'I trust X never to take a bribe, because I know X.' but you can't say 'I trust all members of the profession X is in, because they are in that profession.'
   Saying you don't trust Google is just sensible. Saying you don't trust management at something like Mozilla because they are faceless management, (not that all the things said about choices made inside Mozilla are likely to encourage trust) though a bit generalizing, is also fairly understandable. As such, it's not at all unusual that people are going to hold some distrust for the combination of the two, especially when one of the big drivers of Firefox usage is specifically that it's supposed to be more respecting of privacy than chrome or edge. The user base is already primed to be distrustful of tech companies, and not through paranoia but experience.

I'm not saying manipulation of Mozilla by Google is guaranteed to happen but it's honestly less speculative to expect creepy activity from google, a company for which the business model is 'do sneaky shit on the internet,' than to assume absolutely everything going on is totally trustworthy.