Same here. It opens everytime I start Firefox.
I believe chris05 is right. Look here: http://forum.avast.com/index.php?topic=147820.0
To protect my privacy as best as possible (I know there is no 100%) my Firefox (Version 28.0) settings clear the complete browser history including cookies when I exit Firefox. I also have several Add-Ons like Adblock Plus, BetterPrivacy, Ghostery and NoScript installed. WOT too, which works fine next to AOS. Additionaly I regularly clean the browser history (again including cookies) using CCleaner (I do not touch the registry with CCleaner).
I have not found a way to exclude the AOS cookies from being cleaned. At the moment I prefer to keep it the way it is and live with the AOS tab coming up everytime I start Firefox. I find it easier to close one tab than to fiddle around in several Firefox and Add-On settings.
Still, it would be nice if the avast! team would come up with a solution to stop the tab from opening again even though one cleans the browser history and deletes cookies. Maybe with the next program version.
So far, the new avast! version seems to work great!
System: Windows 8 64-bit, avast! Free Antivirus 2014.9.0.2016, Firefox 28.0 (and coming from Linux I actually like Windows 8)