That one was done with a Microsoft update that incorrectly updated people not using Bing.

How to Remove Bing as Your Default Search Engine and Get Firefox to Work the Way it Used to | eHow.com

• http://www.ehow.com/how_5196325_remove-firefox-work-way-used.html
• http://kb.mozillazine.org/Keyword.URL

Removing the Search Helper Extension and Bing Bar | Troubleshooting | Firefox Help

• http://support.mozilla.com/en-US/kb/Removing%20the%20Search%20Helper%20Extension%20and%20Bing%20Bar

-> Tap ALT key or press F10 to show the Menu Bar

-> go to Help Menu -> select Restart with Add-ons Disabled

Firefox will close then it will open up with just basic Firefox. Now do this:

-> go to View Menu -> Toolbars -> unselect Bing Toolbar

-> go to Tools Menu -> Clear Recent History -> Time range to clear: select "EVERYTHING" -> click Details (small arrow) button -> place Checkmarks on Cookies, Cache -> click Clear Now

-> go to Tools Menu -> Options -> General -> When Firefox starts : select "Show My Home Page" -> Type the address of the website which you want to be your HomePage e.g. http://www.google.com

-> Click OK on Options window

-> click the Favicon on SearchBar -> click Manage Search Engines -> select Google and move it to top -> click OK

-> go to Tools Menu -> Add-ons -> Extensions section -> REMOVE Bing Extension (Add-on) -> Restart Firefox

You can enable your Known & Trustworthy Add-ons and Themes later. Check and tell if its working.