To make Thunderbird uses Firefox as default browser, just add these lines on your all-thunderbird.js.

If you’re using Debian and install Thunderbird from .tar.gz package, look to /usr/local/src/thunderbird/defaults/pref/all-thunderbird.js.


pref("network.protocol-handler.app.http", "/usr/bin/mozilla-firefox");
pref("network.protocol-handler.app.https", "/usr/bin/mozilla-firefox");
pref("network.protocol-handler.app.ftp", "/usr/bin/mozilla-firefox");


Sourced from http://bruno.vernay.free.fr/wp/?p=9