If you download a fresh copy of Firefox, you'll note that the default search engine is not whatever you already have picked in all your other browsers. Instead, Mozilla chooses Firefox's default search engine, and the list of search providers in the accompanying drop down menu, based on their popularity. Thus, Bing isn't one of the choices.
This might seem a bit odd to readers who remember last week
's comments from a Mozilla executive urging Firefox users to ditch Google for Bing due to privacy policies. In order to switch—or even to try the service in the search bar—users have to download the Bing add-on for Firefox
; they can't simply change the drop-down choice.