I'm assuming you're talking about Incredimail's Mystart, and if you are, whew, what a headache. Helped a few users through this process, I swear the thing is like a virus. To the point that people have written uninstall tools they charge money for. Its the way it imbeds into your browser (Firefox in this case)
1. At Firefox address bar, enter about:config and press ENTER.
2. At Filter: Field, type keyword.url
3. You should see a Preference name of keyword.URL in the list. Double click it, a “Enter String value” input box will appear.
4. Replace the string with:
Click “OK” button
Next. Open firefox, go to Tools, Options. Click the general tab. Type a new homepage of your choosing in there. www.google.com for example and then click ok.
Restart your browser/computer.
If that doesn't work you have to go the complicated route here. Ignore the "reason" for removing it, as its purpose is the same for us.http://support.microsoft.com/default.aspx?scid=kb;en-us;297069
Please remember to return and click accept IF my solution resolved your issue. Thank you