Mozilla added Bing as a search engine option in the Firefox 4 browser on both the Mac and PC. Google and Yahoo! are currently the two search engines in Firefox's search engine list and now that Yahoo! is powered by Bing, in the US at least, it makes sense for Bing to sit beside the two.
Mozilla will be following Apple's footsteps too, Bing was recently added as a search option for the Safari web browser.