http://www.ghacks.net/2011/10/27/alternate-firefox-ships-with-bing-big-deal/
http://www.bing.com/community/site_blogs/b/search/archive/2011/10/26/bff.aspx
Last year, we worked with Mozilla to add Bing as a search option that ships with Firefox. Since that time, you have told us to make it even easier to use Bing in Firefox.
http://www.firefoxwithbing.com/
we’re teaming with Mozilla to release Firefox with Bing
Microsoft is neither licensing nor distributing Firefox to you.
http://www.tomshardware.com/news/firefox-with-bing-mozilla-firefox-browser-wars-bing,13858.html