Comment 3 for bug 949949

See these related forum posts:

http://askubuntu.com/questions/112194/kubuntu-12-04-firefox-and-rekong-dont-know-how-to-open-downloaded-files/112820#112820

https://www.linuxquestions.org/questions/slackware-14/make-firefox-aware-of-kde-apps-615686/

Or others that can be found by searching with keywords like "KDE Firefox mime".

In the mean time, here is a possible workaround from Canonical support:

1) stop firefox
2) rm ~/.mozilla/firefox/<profile>/mimeTypes.rdf
3) start firefox
4) download a file
5) open it from the downloads menu and choose the application. Browse to /usr/bin/xdg-open. then check to remember your choice

Now both open and open containing folder should work. You can then change the preferred programs through kde's file associations settings.