Comment 39 for bug 133133

It seems that firefox (and most GTK apps running in kde3 and kde4, such as file-roller and catfish) don't respect kde's file type associations. Instead firefox looks at:
- /usr/share/applications/defaults.list : this is, AFAICT, a static file created when you first install the system

AND

- /usr/share/applications/mimeinfo.cache : this is a dynamically changing file, it's changed when you install a new programme. So for example it will change when you install nautilus or pcmanfm in kde.

Firefox will look first at defaults.list but will give preference to mimeinfo.cache. So if the same entry (e.g. inode/directory) exists in both defaults.list and mimeinfo.cache firefox will take whatever mimeinfo.cache says.

You can manually make "Open containing folder" use your favourite programme by editing /usr/share/applications/mimeinfo.cache . Basically you need to edit the "inode/directory=" entry and put the desired application's .desktop name up front, so to use dolphin, change it to:

inode/directory=kde4-dolphin.desktop;

Notice the extra kde4 in kde4-dolphin.desktop , this is necessary because dolphin.desktop is not in the same folder as mimeinfo.cache; mimeinfo.cache is in /usr/share/applications while dolphin.desktop is in /usr/share/applications/kde4 . If a .desktop file is going to be used and it exists in /usr/share/applications then use its plain name without a prefix.

To use konqueror instead of dolphin it should be:
inode/directory=kde4-kfmclient_dir.desktop;

Be aware though that /usr/share/applications/mimeinfo.cache will be changed automatically when you install a new *relevant* programme/file manager such as nautilus. However you can maintain a local copy that will not be changed when you install anything else, put a copy of defaults.list and mimeinfo.cache in your home folder in ~.local/share/applications.

The problem that existed with dolphin not using the right folder url has vanished in kde 4.2. The problem was that by default the command line in dolphin.desktop was "dolphin %i -caption "%c" %u" and that didn't work in kde 4.1.2 but it works OK with the same command line in KDE 4.2 (I wouldn't take that to the court, though :) ) I know for sure that it's working OK in kde 4.2 .

My system specs:
KDE 4.2
Firefox 3.0.5

A wild guess is that in gnome there's gnome-settings-manager that makes whatever file associations you change in Gnome get applied when you log into Gnome, but gnome-settings-manager isn't run in KDE.