- Attempt #1
First I tried recommended method:
sudo update-alternatives --config x-www-browser
At first I thought it worked fine, but first use of Thunderbird proved me wrong. This command works mainly for KDE applications, which thunderbird is not (it opens Firefox by default).
- Attempt #2
Ok. Let's try to find it somewhere in system settings. I found something like this:
System Settings > KDE Components > Default Applications > Web Browser
This must be it! Oh, it's so easy! No, wait a second - it still doesn't work with Thunderbird :\
- Attempt #3
Following instructions from 'Opera help' for KDE environment I tried to associate .html and .htm files with Opera:
In Koqueror menu bar choose:
Settings > Configure Konqueror > File Associations > text > html
You can set there the order in which the application for a given type should be chosen.
- Attempt #4 (successful!!!)
OK, last try. I'm serious - it's now or never! I switched to Gnome for a moment and set the default browser from there:
System -> Preferences -> Preferred Apllications
Quick switch back to KDE and...
Ladies & Gentleman - we have a WINNER! I mean it works ;)
1. This guy had the same problem.
2. I've found solution here.