There's been speculation for a long time that the real reason Google pays for that is to keep Mozilla afloat and stave off antitrust investigations related to Chrome.
That may have changed with other browsers coming out, although almost all of them are based on Chromium. Ianal, have no idea whether or not someone can be a "competitor" if they're using largely the same source.
If it's true that Google is mostly paying for antitrust avoidance and that something like Edge isn't a "competitor", Mozilla has substantial leverage. Mozilla just needs to be cheaper than an antitrust investigation and potential loss of that case. I would imagine that's not hard to do, that sounds expensive.