Linux – Fixing PDFs opening in GIMP in Firefox/Zotero

I’m not exactly sure why, but with the latest Firefox updates, every time I download a PDF using Firefox or try to open one using Zotero integrated with Firefox, the PDF opens in GIMP.  This didn’t use to happen, but it’s really annoying.  It’s doubly annoying since you can’t solve it inside Firefox.

It would make sense to be able to change this in one of two ways.  First, by simply setting your system-wide preference for what program opens PDFs, that should do it, but mine was already set to Okular.  The second logical solution would be to change the default applications in Firefox, but that doesn’t do anything.  It turns out, the solution is to edit a different file, changing the order of default applications for opening PDFs.  Here’s what you need to do:

  1. Open a terminal and gain root privilege to edit the following file: /usr/share/applications/mimeinfo.cache.  Here’s the command I use in Kubuntu:
    sudo kate /usr/share/applications/mimeinfo.cache

    NOTE: As of Kubuntu 18.04, you now have to use the following command to edit protected files with Kate:

    SUDO_EDITOR=kate sudoedit /usr/share/applications/mimeinfo.cache
  2. Once you have edit permissions for that file, search for this line:
  3. The order of the applications after the “=” indicate the order in which they will be used to load PDFs.  Right now, GIMP will be used first.  All you need to do to fix this is to change the order so Okular is first, like this:
  4. Once you change it, save your changes and then restart Firefox.  Your PDFs should now load in Okular.

(Notes: Nod to this forum post.)