/etc/xul-ext/compactheader.js is in xul-ext-compactheader 2.0.8-1.
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-compactheader in this file.
// You can override here the preferences specified in
// /usr/share/xul-ext/compactheader/defaults/preferences/prefs.js
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