Here is the technique I just used to make mailto links point from
Firefox to Thunderbird. This is for Firefox 1.0 (don't know about any
others):
1) Launch Firefox and view the configuration strings using the URL
about:config
2) using the field at the top, search for
mailto
3) If you do NOT have network.protocol-handler.app.mailto (normally
you don't), create it using these steps:
a) right-click
b) choose New --> String
c) new string: network.protocol-handler.app.mailto
d) new value: /usr/bin/mozilla-thunderbird