Open Links from Thunderbird in Firefox
In Debian 3.1, Thunderbird does not launch Firefox if you click on a link in an email message by default. There is no option to change this option from within Thunderbird itself.
Instead, close the email client, fire up an xterm or konsole and type:
find ~/ -name prefs.js | grep thunderbird
This will find the full path to the file that holds the Thunderbird preferences on your machine.
Now you know where it is (the path varies on everyone’s machine for security reasons) edit it and add the following line:
user_pref("network.protocol-handler.app.http", "/usr/bin/firefox");
Restart Thunderbird. Now when you click on a link in an email it should open in your favourite browser.
(Adapted from Linux Basics tutorial.)