/etc/xul-ext/firetray.js is in xul-ext-firetray 0.5.3~rc1+dfsg-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-firetray in this file.
// You can override here the preferences specified in
// /usr/share/xul-ext/firetray/defaults/preferences/prefs.js
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