Fixed for me

I tried several options in this thread, and this worked for me:

The correct way to get Incredimail Mystart out of Firefox is:
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: http://www.google.com.my/search?q= Click "OK" button
For Internet Explorer, you just install Google Toolbar again, and Mystart goes away.

Do that AND the following:

The correct way to get Incredimail Mystart out of Firefox is:
1. At Firefox address bar, enter about:config and press ENTER.
2. At Filter: field, type mystart
3. At least 1 entry will appear, right click, click RESET
-
4. At filter: field, type incredibar
5. At least 1 entry will appear, right click, click RESET (for me there were several, about 25 or so entries)

After doing this, everything appeared to work normally. Hopefully nothing crops back up.