Bing released a Firefox With Bing addon, which prompted Scott Hanselman to tweet a bit:
Looking at the addon code, it was released under MPL 1.1/GPL 2.0/LGPL 2.1 so...
In response to the peoples' demand, I present to you
Firefox With Hanselman
(Click to get the addon.)
This addon does pretty much exactly what the Bing addon does:
Free. No warranty expressed or implied. Works on my machine, etc.