I have same problem with this random .exe executed from the avast update,

If i let to pass this application with my firewall then the save as option in Firefox dont work anymore, and also all the bottons from the tool/setup from firefox that need to open a Examine…

Using a restore point from Windows and reinstalling firefox saving my profiles, i get again firefox working right, but if i let to work this emergency update process from Avast again firefox fails!

Any idea about this?