When updating your default browser in Gnome/Ubuntu, HTTP and HTTPS are not equal.

Running two browsers for a variety of reasons (Firefox, primary, Chrome or Chromium, secondary) on all my platforms meant I kept the habit up when I started using Linux more frequently.
My main machine (both work and home) is now Ubuntu 10.10 (likely to be upgraded further soon when 10.10 goes out of support) and though i'm also fiddling with Linux Mint, i'm not using it fulltime yet... (but I digress).

Recently I've noticed somewhat annoying behavior where having received an email with embedded links into Thunderbird, clicking on supplied links would variously use either Firefox or Chromium without apparent pattern.

Eventually I worked out that http:// links were opening in Firefox and https:// links were opening in Chromium.

In frustration (as everywhere I could see, 'default browser' was set as Firefox) I posted to NZLUG and a very helpful post this afternoon revealed the winner:

Use gconf-editor and check the following:

Whaddiyaknow... default http handler was indeed 'firefox %s' and default https handler was 'sensible-browser %s'.

Running sensible-browser called Chromium. (what a conceited name to use!)

Changing it to Firefox appears to have put things back where I'd expect them to be; Firefox is my default, and Chromium has particular uses.

The relative obscurity of some of these fixes is occasionally somewhat frustrating, so i'm publishing this in the hope it'll benefit others. I've also put it up on Ubuntuforums.org attached to another post with similar symptoms, albeit a different fix.