Firefox freeze while finishing a download

Hi, I am using Avast Free (5.0.677), with only the File system shield active. I have a problem with Firefox 4.08pre where the browser freezes when a download completes. The “browser.download.manager.scanWhenDone” entry is set to false in Firefox. I have even tried excluding both the Minefield installation directory and the active profile directory in the File system shield Expert settings. The problem goes away, when I disable the shields temporarily when a file is downloading. Is this a common problem or is it a problem with my configuration? I am using Windows 7 RC1 on a Dell Inspiron laptop. I don’t use any other security software and my firewall is Windows Firewall with Advanced Security.

Does it crash/slowdown older versions of FireFox? Like, 3.6?