Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If you auto-updated to 15, you are on the wrong update channel so you are getting a test version. I think the easiest way off of this is to go to the Firefox page and download version 14. Install it, and it should keep all of your bookmarks, settings, etc, but get you off of the "update to test builds" cycle.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.