/etc/xul-ext/custom-tab-width.js is in xul-ext-custom-tab-width 1.0.1-2.
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-custom-tab-width in this file.
// You can override here the preferences specified in
// /usr/share/xul-ext/custom-tab-width/defaults/preferences/defaults.js
|