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.
The problem:
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.
The fix:
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
kcmshell defaultapplication
Sometimes I miss the times of simple configuration files.