Seems Microsoft’s Bing is not popular enough for Firefox users, according to a Mozilla exec. That’s why an add-on is required to add Bing as a search option from within Firefox. Another home-work for Microsoft’s search engine to fight against Google.
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.