I think I may know what happened with my original install. When I ran the installer I had FF open so the installer asked me to close it. Rather than clicking the close FF button in the Avast dialog, I just brought FF to the front and quit it. As soon as I did Avast continued with the install. I just installed Avast on my MacBook AIR but I closed FF before I started the installer and the CA was imported into FF properly without me having to do it manually.

Just a thought.