Why does everyone assume that Google is the only game in town for funding Firefox?
Firefox has 1 out of 5 users on the internet. If Google lost Firefox it would be significant. They really wouldn't want to lose it.
Microsoft would love to have Firefox's default search area (and love taking it from Google) and they've repeatedly demonstrated their ability and desire to throw gobs of money at a product until it succeeds (or clearly fails beyond redemption) - and Bing is one of those products.
It seems like few people made the connection when Microsoft partnered with Mozilla to release a version of Firefox "powered by Bing search". Just a month or so later Mozilla announced their three year, $900 million search deal with Google. It seems clear that Mozilla either just before, or during, negotiations with Google demonstrated their ability to pull the trigger with Microsoft.
Google does not want to lose Firefox, but further would hate to lose it to MS. Either way, even if they did, Mozilla would not go hungry.