Me default djvu viewer is okular:
$ xdg-mime query default image/vnd.djvu okular.desktop
[Added Associations] image/vnd.djvu=okular.desktop;evince.desktop;
When I click on a djvu -- okular opens it.
/usr/share/applications/defaults.listlinks to this file):
However firefox thinks my default app for djvus is evince:
How do I set okular as a default viewer in firefox? [this doesn't mean I want "Use okular" to be a default action -- I just want it to be a default viewer]