the script shield also does bad things with flash video and FF12.
The solution is to remove all traces of Mozilla Firefox 12 from your system and reinstall version 11 this is at your own risk. At least until Mozilla and Avast have sorted out their issues.
This is not ideal from a security perspective but it works for my situation and may suit you if you know what you are doing.
Backup your bookmarks and passwords only from your profle using Mozbackup first. Do not back up extensions and addons.
Get the older version from Firefox 11 here,
http://download.mozilla.org/?product=firefox-11.0&os=win&lang=en-USRemove All traces of FF12 from your Windows user profile and Program Folders on your system.
Temporarily pause the script shield and Install FF 11 and restore only your bookmarks and passwords with Mozbackup. Do not restore anything else or you will have to start again.
Once Firefox is installed and up and running restore your bookmarks and passwords, reinstall your addons and extensions fresh.
This has worked for me.
Cheers