@JamesD,
In the registry old choice are preserved when installing K-Meleon. It's not an error to think that, when uninstalling K-Meleon, the old values would be restored, but I'm not sure the setup can do it.
If it's not the case (I'm thinking the other browers don't make better), you're right to say that the choosen browser must have the way to do it (by choosing an option to make it by default).
A+
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Web:
http://jujuland.pagesperso-orange.fr/
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc