GuruStop.NET

1. Go to https://addons.mozilla.org/en-US/firefox/addon/10434 ( Update: This is the only official provider made by Microsoft itself)
   This is the URL I originally posted (from which the below screenshots come): https://addons.mozilla.org/en-US/firefox/addon/12205 – Of course you are recommended to go to the official URL above.
   
2. Check “Let me install this experimental add-on” and click “Add to FireFox”
   
   Now the add-in is there.
3. In the search bar, click the little triangle at the left and choose Bing.
   
4. In your browser it may not the first item nor bold as in the pic here. Click “Manage Search Engines…”.
    
5. Click “Move Up” to have it the first item
   
6. If you have Google Toolbar, you need to check more settings:

1. Go to Google Settings. In the “Layout” tab, make sure to choose “Classic Toolbar Layout”. Save that.
   
2. In Firefox address bar, type:
   about:config
   without “http://” or anything, and click the button that appears then.
   
3. In the “Filter” field type: “browser.search.defaultenginename”
   
4. Double click the only result that will show, you get a message box where you should  enter text. Replace the existing text “Google” with “Bing” instead, and click OK.
   
5. Close the window. Your default provider is set.

Have fun :-)