Microsoft would love to, but they can't afford it. Bing is already losing huge amounts of money, and doing yet another deal for market share isn't going to be popular within MS unless it is profitable.

While MS could make money on a Firefox deal, they can't nearly make as much as Google can simply because Microsoft can't monetize the users as well as Google can.

You can see this on the demand side - many people try out new keyword advertising campaigns on Bing because they are so much cheaper than on Google.

Once they are confident they have a good set of keywords, they switch straight to Google (and at best keep the Bing campaign going). They'll pay more at Google, but get so much more traffic it is worth it.