Few years back when Mozilla and Yahoo! agreed to use Yahoo! as default search engine in certain markets, there were strong complaints. That issue reaffirmed one thing: Google was one of the very few big companies loved/endorsed by open-source extremists.
It is well known that Mozilla's multi-million dollar lucrative contract with Google is about for renewal, and the open-source browser developer has been working on a browser that uses rival Microsoft's Bing as default search engine.
The customised version of Firefox - Firefox with Bing - can be downloaded from firefoxwithbing.com.
And to keep those who can't live without latest of whatsoever a peace of mind, Firefox with Bing uses the latest Firefox version.
Unsurprisingly, Mozilla has been careful on wording about this issue.
for more detail, Mozilla, Microsoft debut Bing-ed Firefox (27 October 2011)