Firefox secrets

Howto make thunderbird the default mailto program when clicking an address with a mailto symbol (a little letter)

Type: about:config in the URL-field of firefox
You should have a line: user_pref("network.protocol-handler.app.mailto", "thunderbird");

If not you can add one by "clicking right mouse" in the "about window" and adding a new string value: preference "network.protocol-handler.app.mailto"

value "thunderbird"

How to learn about firefoxes active plugins

Type: about:plugins in the URL-field of firefox