These items do automatic self-updates without user's control, notifications, or consent (generally). These guys check back with their corporate masters over unencrypted http.
Due to no ssl certs or anything like that then this leaves users using these extensions in a dangerous position because of things like DNS spoofing. There is no checking of hosts... It's all based on domain names, which is known problem.
If your using the community contributed stuff from addons.mozilla.org and whatever whitelist Firefox has setup by default then your fine. These things are (generally) immune to this sort of problem.
