My system did an auto update of Firefox v28.0, now EVERYTIME the browser is loaded it launches my default page in one tab and an avast! Online Security page (http://aos.avast.com/upgrade/2006) in another tab.
Drove me nuts, too! Most of us clear out our cookies. And that’s what causes it. I use CCleaner (free edition) and was able to eliminate it. In CCleaner go to Options, click on Cookies, then move