Firefox/Mozilla is not as reliant on Google anymore as it once was, having diversified which search engine gets set in which area. There would also still be a reason for google to be the default search engine even in a browser that blocks third party trackers by default, namely the original reason of getting users to use your search engine. So I don't see a problem here and I doubt Google would or could force Mozilla to not make that change, even though the business people in Google probably don't like it.