Thunderbird, Firefox. Firefox, Thunderbird

Once you've gotten everything set up with Thunderbird as your emailer, you need to get it talking nicely to Firefox, and visa versa. That is, when you have an HTML link in Thunderbird, clicking on it should bring up the link in Firefox. And clicking on a mailto link in Firefox should bring up a composition window in Thunderbird.

Doing this while running a GNOME or GNOME compatible desktop such as FVWM is easy:

• From a text window run
  $ gnome-control-center
• Click on Preferred Applications
• Click on the Internet tab.
• Choose the default web browser and email client from the menus, or type the address in the box, if needed.

I suppose there's a similar thing you can do under KDE, but I'm not all that familiar with the system.

You can also hard-wire the behavior into your browser by editing the user.js files. That is, edit

~/.mozilla/firefox/xxx.default/user.js

so that it contains the line

user_pref("network.protocol-handler.app.http","/usr/bin/firefox");

and edit

~/.mozilla-thunderbird/xxx.default/user.js

so that it contains the line

user_pref("network.protocol-handler.app.mailto","/usr/bin/mozilla-thunderbird");

(xxx.default is actually a random string assigned by Firefox and Thunderbird)

This marries Firefox and Thunderbird, they're linked together, and GNOME cannot pull them asunder.