I had a very similar redirect virus and I went through all the things you have mentioned above. Scan with this, check with that.. The solution I found to the problem was to just Reset Firefox back to default settings. Mozilla has a walkthrough
http://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems or you can access it by typing about:support in your firefox address bar. There is a button that says Reset Firefox as well as alot of system information in case you just like looking at stats.