It can't be installed unless you have an xpi file for it and that isn't in the firefox/mozilla add-ons site, you have no settings in firefox, it can't be updated in firefox, it can't be saved in firefox using something like FEBEs add-on.
Effectively it isn't stand alone (you have little control over it) and isn't being offered as such. Offering it as a stand alone add-on won't show the strength of avast as it currently has nothing to do with security, but reputation alone and that from 'avast' users.
So if they want to see the power of avast, they only have to try it as the web shield and network shields are of a much greater importance.
