system
11
This was annoying me too, so I decided to try and get rid of it.
First I tried GreaseMonkey, but I found that the scripts do not have permission to close the web page.
Then I tried writing an Extension (add-on), it seems to work for me on Linux and Windows
Note that the tab still loads, but the extension blows it away before too long.
Here is a link to it, has not yet been reviewed at the moment: https://addons.mozilla.org/en-US/firefox/addon/auto-close-tab/
If you try it, after installing edit the config and put in the avast url to autoclose, i.e. http://aos.avast.com/upgrade/2006