And the risk of Mozilla switching to Bing by default is the reason that Google is so eager to pony up that $300 million every year. Thanks to Microsoft, Mozilla is the one with leverage in this relationship.

Microsoft is probably eager to pay more than Google to be Firefox's default search engine. I believe Microsoft and Yahoo already pay Mozilla to be included as non-default search engines. So even if Google fans switched from default Bing to secondary Google, Mozilla would probably still make money from Google.