You probably have the babylon toolbar installed somehow. Go to Tools > Add-Ons and uninstall it. You may have to uninstall from the control panel as well.

I have this same problem its very annoying I am constantly having to confirm that I want the change I made to stay if I don't then it goes back to the previous search engine. I do not have Babylon Toolbar installed. I went to Tools > Add-Ons and it wasn't there.

problem starts when you edit default search engine list.

temporary solution: click the search engine selector pulldown and click manage search engines. click restore defaults. unfortunately you now have to wade through the list of possibilities again but will stop getting alerts.

Mozilla please fix this-only a temporary solution for those who do not want the default list.