Technically nothing; the problem lies entirely in the Mozilla Foundation which takes about $500B each year from Google. Officially the motivation is to keep Google as preferred search engine, but seriously the Firefox user base is so small that I don't even think Google would be interested for much less if there weren't strings attached that very likely create a huge conflict of interest if Mozilla become dependent on that money for their own survival.

https://www.bloomberg.com/news/newsletters/2023-05-05/why-go...