How can I be sure that a personna or theme does not have a virus attached?
There are numerous themes and personnas to pick from, but they are not managed by Firefox. The chances of acquiring a virus from one of these is probable, so I'm wondering if Mozilla uses a filter for these features.
Personas (aka Lightweight Themes) are screened by Addons staff before they are made public. "Heavy" Themes are screened by A-V software when they are initially uploaded and are marked as "experimental", like this one - https://addons.mozilla.org/en-US/firefox/addon/10891/ - until they pass a review process done by Addons staff.