Why would Google fund a competitor? On face value that doesn’t make much sense, but megacorps with so many tendrils rarely make cohesive decisions.

90% or so of revenue is generated by search engine royalties, i.e. Mozilla getting paid for the choice of the default search engine (mostly Google).

It doesn't "fund" it - it buys its position as the default search engine in Firefox, which makes up the bulk of Mozilla's income. In this theoretical world where Google actually goes away, presumably some other company would want to pay for that position.