Strange. I've ran Avast (with default settings) in Windows 7 Professional with the latest 3 version (I think 3.26) Friday, and with the current 10 version now, and I've got no problems at all.
Maybe it's some Firefox add-on you have that causes the problem?
I'm using avast version 6 and latest version of firefox. After upgrading ff to version 4 when I launch ff it cause a complete freeze of windows (7 ultimate) and then bsod comes up. I disabled avast and the problem solved. Though I mainly use ie9 for browsing stuff but I'm curious to know what is the cause of this error.