Ugh .. sorry but that's the worst suggestion so far. No image the
goes to http://addons.mozilla.org/
to install addons ... it won't
work. (just one random example but you get the idea).
I imagine the user would change the start page to about:blank, then
open the firewall, then everything works as normal.
Of course, this only works for single-user machines. A multi-user
machine wouldn't protect the second user to run Firefox.