Kubuntu 12.04 is no longer a Canonical-maintained distribution. As such, support for the firefox-kde-support package has been discontinued.
Here’s a workaround to get firefox’s downloaded file handling to work correctly:
Here is a workaround for the Firefox problem from Canonical support:
1) stop firefox
2) rm ~/.mozilla/firefox//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