Comment 15 for bug 1551171

Ok, did some more investigations, when I copy the corresponding line containing for example pdf to $HOME/.local/share/applications/mimeapps.list then those settings are honored by firefox, when this is not found in this file then only the mimeinfo.cache is used. So, yes, firefox honors the user's settings if it is in that list.
Still, it would be nice to use something more desktop independent. Chromium uses xdg and that works fine for me, no extra configuring, it takes my user's kde's default apps without a problem. The problem with the gnome way is that when you don't have gnome installed you have to add all the required apps to this mimeapps.list manually because KDE only writes those apps to this list that are manually changed by the user's kde configuration when this is different from the system kde default. xdg has no problems in this respect.