Inspired by Mozilla Firefox Bug 727422, comment 30, I found out that Firefox is currently (FF 36.0, Ubuntu 14.10 with LXDE) using the org.freedesktop.FileManager1-Interface on the Dbus session bus. As stated in the specification of this interface, Nautilus (also known as Files) is the only file manager that implements this interface. Therefore, Firefox will always choose Nautilus as file manager if it is installed.
So there are two possibilities to resolve this first problem:
- Uninstall Nautilus/Files
- Tamper with the org.freedesktop.FileManager1-Interface that Nautilus offers to disable this method of Firefox to open a file manager. Therefore, you could just delete
/usr/share/dbus-1/services/org.freedesktop.FileManager1.service. But maybe, it is a better approach to use the proposed solution of https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/133133/comments/74 and create
~/.local/share/dbus-1/services/org.freedesktop.FileManager1.service and set its exec-line to
/usr/bin/false (just copy
/usr/share/dbus-1/services/org.freedesktop.FileManager1.service there and change it). This avoids deleting/editing a file created by your package manager and you do not need super user privileges. After having done so, you possibly need a restart.
As soon as Firefox can not use the Dbus-Interface anymore, it will fall back to a second way to figure out which file manager to start. This will lead to the second problem:
The default applications should be configured with several different
mimeapps.list-files with different priorities. It seems to me that Firefox is correctly consulting the files
/usr/share/applications/mimeapps.list but not the deprecated
defaults.list-file. So, you have to add the
inode/directory=pcmanfm.desktop-entry to one of the three files mentioned above. This can also be done by calling
gvfs-mime --set inode/directory pcmanfm.desktop or
xdg-mime default pcmanfm.desktop inode/directory.
Lxde itself seems to use
~/.config/lxsession/LXDE/autostart to determine its default file manager. Therefore, you have to edit one of those files manually.