/usr/share/xul-ext/custom-tab-width/defaults/preferences/000system.js is in xul-ext-custom-tab-width 1.0.1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/xul-ext/custom-tab-width.js
/etc/xul-ext/custom-tab-width.js