It seems Mozilla Thunderbird (GREAT mail client, BTW) seems to download and open the downloaded HTML file when you click on a URL in an email. This is usually not what I want… so:
Exit Thunderbird if it’s running.
Edit your prefs.js (usually in ~/.mozilla-thunderbird/default/random_characters/prefs.js
Add this line: user_pref(“network.protocol-handler.app.http”, “/usr/bin/mozilla-firefox”);
Obviously, if you don’t have an intelligent mozilla-firefox script in /usr/bin that is able to make use of a running instance of firefox, you might have to use the mozilla-firefox-xremote-client or somesuch. Perhaps you even prefer to use another browser… heaven forbid!
For more details, see here: http://www.mozilla.org/projects/thunderbird/linuxurls.html