Bing keeps setting it's self as the default browser

Ever since I set Bing as my home page it won't let me set any other search engine or web page as my home page. Whenever I set another page it reverts to Bing when I start Firefox. Whenever I go to "tools" "options" and set a new page it reverts to Bing when I restart Firefox . Whenever I drag from the left of the address bar to the home page button it sets the current web page as my home page until I exit Firefox. When I restart Firefox it"s back to Bing. In "manage search engine list ". Bing is not listed. I have run a virus scan using "Norton Internet Security" and it has found no problems.