Set "Open containing folder" to use Dolphin and PDF files to use Okular in Firefox on Kubuntu

If you, like me, are tired of having Firefox open up Nautilus for folders and Evince for PDFs in KDE on Ubuntu, you might want to change your default application settings. To fix this once and for all, open up /usr/share/applications/defaults.list and change the line with

application/pdf=evince.desktop

to

application/pdf=kde4/okular.desktop

Then, to set up Dolphin, change

inode/directory=nautilus.desktop

to

inode/directory=kde4/dolphin.desktop

And there you are! Courtesy of Jeremy’s comment on this post.