Integrate Firefox and Thunderbird in KDE

Whenever you click a http:// or mailto: link in KDE, by default, Konqueror and KMail will open. I’m not saying these are bad apps, just that if you have Firefox and Thunderbird installed, why wouldn’t you want to use them fully?

Why not make them open those type of links and even more, why not make them look like a KDE application? But let’s take one step at a time.

How to make Firefox and Thunderbird the default web browsing and mail client?