Comment 32 for bug 1551171

(In reply to Eevee (Alex Munroe) [:eevee] from comment #26)
> tl;dr of comment 24: if you're on KDE, and Firefox isn't opening e.g. PDFs
> or directories with Dolphin or Okular, it's neither a Firefox nor a GIO
> problem.

This is not exactly the case, especially considering opening directories with dolphin.

It turns out that FF (tested on 29.0.1 on Gentoo) when built with dbus support has a wrong way to do it.
It first calls (instead of checking if exists first and then calling) org.freedesktop.FileManager1 on the session bus and thus if nautilus is installed it is started as "/usr/bin/nautilus --no-default-window" and in this way FF doesn't honor the XDG at all.

So for now a workaround is either to delete the /usr/share/dbus-1/services/org.freedesktop.FileManager1.service or change its Exec to "dolphin" (if you are under KDE, though that may have implications 'cause dolphin doesn't register any org.freedesktop.FileManager1)