This is one of the simple things which required more searching than I expected. I note the solution here in the hope that it is helpful to the next person searching for an answer.
We’re running plain Ubuntu installations (gnome) with Kmail (for Kolab). Clicking on links in emails opens a shell window with Lynx and not Firefox.
Most of the search hits I found advised me to use kcontrol and go to “KDE Components -> Component Chooser”. Well, that option isn’t enabled in the Ubuntu version of the KDE packages. As I don’t run the KDE panel, the suggested solution doesn’t work either.
The solution is contained in these bug tracker messages:
sudo apt-get install kde-systemsettings
Sometimes I miss the times of simple configuration files.