Comment 16 for bug 1196339

I am running into the same issues using kubuntu 15.04 right now. The problem there is that FF opens gwenview instead of dolphin.

It seems (as also already pointed out on the mozilla bug tracker) that the /usr/local/applications/mimeinfo.cache contains the wrong preference for inode/directory

$ cat /usr/share/applications/mimeinfo.cache | grep directory
inode/directory=gwenview.desktop;kde4-dolphin.desktop;

If I switch the order of gwenview.desktop and kde4-dolphin.desktop within the cache file, FF immediately does not open the wrong application any more.
But when I run the mimecache update script like this:

sudo update-desktop-database /usr/share/applications/

the wrong order gets written again and FF shows the same behaviour as before.

The strange thing is that the Preference seems to be alright...

$ cd /usr/share/applications/
$ grep InitialPreference -A 2 kde4/dolphin.desktop gwenview.desktop
kde4/dolphin.desktop:InitialPreference=10
--
gwenview.desktop:# InitialPreference should be greater than Okular so that Gwenview is the
gwenview.desktop-# primary application associated with images, but less than Konqueror or Dolphin
gwenview.desktop-# so that Gwenview is not the primary applications for folders.
gwenview.desktop:InitialPreference=8

So even though a higher preference is given to dolphin, gwenview is what ends up first in the list.