I am surprised by this. Mozilla is literally pumping out good open source software and no other company except Google is backing them?
Two points: Mozilla is pumping out mostly user facing open source software, which will probably attract less money than, say, Red Hat. Also, it's not that no company except Google is backing them. It's that they get a huge amount of money from their search box - currently paid by Google. Microsoft might well be happy to become the default and pay just as much.
Google does consider the search engine placement valuable but it is probably also funding Mozilla as a hedge against anti-trust lawsuits.