Comment 40 for bug 52670

I have a workaround to make Thunderbird and Firefox use KDE's settings.

First, create a script called /usr/local/bin/kfmclientexec that contains these two lines only:
#!/bin/sh
kfmclient exec "$*"

Save and make it executable (+x).

Next, in Firefox, go to about:config and create or edit a string value called "network.protocol-handler.app.mailto" and set it to "/usr/local/bin/kfmclientexec". This should make Firefox use the e-mail client you configured in KDE.

In Thunderbird, go to the about:config screen (Edit --> Preferences, Advanced, General, Config Editor) and create or set two string values called "network.protocol-handler.app.http" and "network.protocol-handler.app.https" to "/usr/bin/kfmclientexec"

This should work with the default settings in both programs. I hope this works, at least until things get sorted out better. :) It works for me, at least.