It’s been driving me mad for months why sometimes when I click on a URL in Thunderbird it opens it in Firefox, and sometimes it opens it in Epiphany or Galeon or something like that (even though I use KDE). I couldn’t see a pattern – was it to do with whether it was a body URL or in the header? Was it particular sites (Bugzilla seemed never to work)? I finally figured it out; it’s down to this small but important sentence in the instructions:
Repeat [this configuration change] for each protocol you want Thunderbird to dispatch (https, ftp, etc.).
So http:// URLs were being correctly dispatched, but https:// ones (including, of course, Bugzilla) were not. If anyone else is similarly incapable of following instructions, perhaps this blog post might help them.