"Users of the popular IncrediMail may have noticed that the service has modified the default search engine and homepage of the Firefox web browser after installation. The free version of IncrediMail will make those changes to the Internet browser unless you select the custom installation option during program installation and uncheck the “Make MyStart by IncrediMail my default search engine” and “Make MyStart by IncrediMail my homepage”.
"If you do not, you end up with a new home page that opens up in the browser and a new default search engine. Let me show you how you can reverse the changes if you have missed to uncheck the options during the installation of the mail program for Windows. …"
Source: Ghacks Technology Newsletter
How to remove IncrediMail MyStart from the Firefox browser