/usr/share/ubuntu-mate/settings-overlay/config/plank/dock1/launchers/firefox.dockitem is in ubuntu-mate-default-settings 16.04.5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [PlankDockItemPreferences]
Launcher=file:///usr/share/applications/firefox.desktop
|