You have to be vigilant. Every time you download add-ons or programs from the Net, the manufacturer attempts to get you to agree to adding their fucking stupid scam tool bar or making them your scam default whatever-the-shit. It pays to spot their little scams and uncheck whatever box supposedly gets you to agree to their little exploits when you install the program. No doubt at some point, you agreed to make that asinine ad-ware scam Bing your default search engine.
So this is how you kill it: Go to the top right hand corner of the screen and click on the 3 weird horizontal lines logo. This is sort of an "everything else you might want to do to the Firefox program" link. Click on "options". Click on the "search" tab. Then eradicate Bing from your list of search engines and insert Google as the default.
Let me know how it works out for you.