Bing is not popular enough for Firefox.

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.

Source -
Mozilla exec: Bing is not popular enough for Firefox

As great an effort as Microsoft has put into creating their Windows Live "empire", Google is just too great a competition. Google is THE definitive website. For many, Google is synonymous with the Internet. Perhaps Bing will gain momentum eventually. For now it will serve as a nice module of the Windows Live system, and will get 99% of its traffic from IE users who didn't bother to change their default search engine.

It is easy to add Bing to Firefox:

https://addons.mozilla.org/en-US/firefox/addon/10434

Coolness said:

But Its Not Google!!
BING.

Bing Is Not Google
Bing Is No Good (except their video search...that's pretty neat how you can just hover over the thumbnail and the clip starts playing...)