Pretty cool actually, just click on the drop down. Then click "Manage Search Engines" Then click get more search engines Then scroll down a bit and find bing or look here: https://addons.mozilla.org/en-US/firefox/addons/policy/0/10434/54407 Then install.
Thanks for the heads up. Not certain I'll even bother with it though. People will leave Bing when its putting out results like it does for: Atlanta Real Estate http://www.bing.com/search?q=atlanta+real+estate&go=&form=QBLH (see how many times that 1 site with its various subdomains is listed?) Bing LOVES keyword laden URLs and on-page (titles, etc). Spammers will quickly flood the search results - and drive away the consumers.