> Does this mean I can access a .onion address from within firefox when I've got the Tor plugin enabled?
Yes.
You will however need to run the Tor software -- Privoxy or Vidalia bundle or... something. I don't think the plugin can do the routing itself.
I haven't used Tor in a while, but there used to be wikis, forums, even shell account servers and search engines available as hidden services. Don't expect anything interesting though.
Tor already provides that https://secure.wikimedia.org/wikipedia/en/wiki/Tor_%28anonym...
> users aren't going to install a new piece of software or a special browser to access masked domains.
Sure they will, just like they install BitTorrent clients. It's only a matter of proper incentive.