Looks like the update didn’t went as it should, or Windows has a problem.
Try this:

  • remove Avast using the uninstall util from the website
  • reboot
  • reboot to a dos prompt
  • run “scanreg /fix”
  • reboot
  • Install Avast
After I uninstalled avast and rebooted (several times, just for fun) Firefox would no longer work.
That could indicate a Windows problem.