Comment 34 for bug 1551171

TL;DR: firefox should do exactly the same as `xdg-mime query default`

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I don’t really care how GNOME does things, and I don’t care about GIO.

If

    $ xdg-mime query default application/pdf

returns

    okularApplication_pdf.desktop

Then I consider my system appropriately configured to open pdf files with Okular, and firefox not respecting this is a bug.

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To help debug this:

The above query worked like displayed even while my ~/.local/share/applications/mimeapps.list and ~/.config/mimeapps.list contained no entry for application/pdf

Which probably means that something in the XDG spec other than mimeapps.list defines the order of default applications.

Firefox however opened PDFs with Inkscape.

Manually adding this line to one of the mimeapps.list files:

    application/pdf=okularApplication_pdf.desktop;

…made firefox open PDFs with Okular