Comment 89 for bug 133133

If you have nautilus installed this becomes a problem because firefox wants to use dbus engine. Firefox apparently uses the dbus engine first to launch org.freedesktop.FileManager1 which nautilus is currently the only app that gets launched/used this way. You have to install your own org.freedesktop.FileManager1.service file in ~/.local/share/dbus-1/services

Also look to see if you have other FileManager1 filemanagers registered to dbus. I had two custom Nemo and Caja, and i just copied those files from their default location to my home directory location and changed the Exec. Firefox gives up, and goes the xdg route - which I thought was supposed to be the standard. This is a complete mess.

This link describes the process in detail: http://debian.distrosfaqs.org/debian-user/iceweasel-and-dolphin/ and https://bugzilla.mozilla.org/show_bug.cgi?id=1106916