Firefox came into existence because Microsoft used their signature anti-competitive business practice to butcher Netscape. Google helped Firefox to generate revenue so as to create a competitor to Microsoft's Internet Explorer and bring to the the Web an alternative to what Microsoft offered.
Firefox used Google as its default search engine. The Google-Firefox is due for a renewal, and Firefox may be desperate to get some revenue, going with Microsoft's Bing is to me going agaist the grain of their existence.The open source browser signed a deal with Microsoft recently to also use Bing as a search option.
Just today the open source browser announced another deal. The Mozilla foundation has announced a version of Firefox with Bing as the default search engine.
Google offers Bing as an option to be selected in the very first step when you install Google Chrome, Microsoft has made it almost impossible to set Google as the default search engine.
Firefox is fast losing market share to Google Chrome due to its sluggish performance. There are users who are still using Firefox for 'philosophical' reasons. The deal and release of the 'exclusive' Bing version breaks that trust in Firefox. Microsoft has a kind of Midas Touch, every company or project it touches simply dies, whether it be Novell, Nortel, Nokia or Yahoo!
Firefox has made a fatal mistake, in my humble opinion, becoming a bed mate to Microsoft.