This will make Firefox “Open Containing Folder” obey the settings you have set in your Desktop Environment (KDE, GNOME, anything else that pays attention to freedesktop.org to open folders with your preferred application (Dolphin, Konqueror, Nautilus, etc):
mkdir -p /usr/local/share/applications/xdg
cat <<EOF >/usr/local/share/applications/xdg/open.desktop
Open /usr/local/share/applications/defaults.list with your favourite editor. Find the line that contains:
If it’s not in the there, add it. Then directly below that line, add this line:
So now the file has this in it::
(And maybe some other items below it if the file already existed).
Now you can right click on the downloads in Firefox and choose “Open Containing Folder” to have the folder open with the correct file browser. You may have to wait a few moments or restart your desktop environment so it notices the change.