Mozilla isn’t dependent on Google search revenue. Mozilla is, (and
it’s a bit concerning, but not terribly so,) dependent on the search
advertising marketplace that most of the web is dependent upon.
Any search provider would, (and others besides Google do) pay for
traffic that Firefox generates for them. Google is the lion’s share
of Mozilla’s revenue not because they pay more to Mozilla for
searches than the other providers, but because there are a whole lot
more searches to pay for.
That’s because Google is the default. If, for example, Yahoo was the
default, it would be the lion’s share of Mozilla’s revenue.
It’s undoubtedly true that Yahoo might be willing to pay top dollar today to replace Google as the default Firefox search engine. But will Yahoo be in a position to do this, say, five years from now? Who else other than Yahoo would pay as much as Google? Microsoft could, but would they?