> Why would you pay a direct competitor to exist when the amount of money you put in to them is an incredibly large majority of their revenue?
One plausible reason is that Google is not a monolith: the entity paying the money is the Search division, which does not compete with Firefox. Chrome does, but Chrome is not a party to the deal. Search is interested in maximizing their own profit, not in maximizing Chrome's usage share.
> Firefox continues to become more irrelevant, and the amount of their revenue they get from Google doesn't really reflect that trend.
It is a revenue share deal. Firefox gets some specific (agreed) percentage of the search revenue from searches done via the Firefox search bar. If the revenue Firefox gets is stable despite reduced relevance, it is because the revenue is stable.
Relevance doesn't translate to revenue. What matters is the number of users and revenue per user.