/etc/xul-ext/lightning.js is in xul-ext-lightning 1.3+build1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | // Place your preferences for xul-ext-lightning in this file.
// You can override here the preferences specified in
// /usr/lib/xul-ext/lightning/defaults/preferences/lightning.js
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