From MozillaZine Knowledge Base

When you click on a web link in Thunderbird, it invokes the default web browser for the operating system. It is possible to change this behavior by changing the default browser.

On Linux, another way (tried with TB1.5 under Suse and Debian) is to insert the following line in Thunderbird's prefs.js or user.js file [1] (of course put in the correct path to your firefox):

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

See also

• network.protocol-handler.app.(protocol)