check your firefox app and see if you have an ask app, toolbar or plugin. get rid of it. Also go to the operating systems add/remove program (cannot be specific since no os was mention) and make sure nothing from ask is there.
you may also be able to remove it by doing a system restore to a restore point prior to when it installed.
The "Ask" folks surreptitiously managed to replace my default Firefox Google Search engine with Ask..
With Firefox's hazy procedure stated in its Tools> Options, I am not able to restore Google as my search engine.