/etc/xul-ext/tabmixplus.js is in xul-ext-tabmixplus 0.5.0.0-1~deb8u1.
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-tabmixplus in this file.
// You can override here the preferences specified in
// /usr/share/xul-ext/tabmixplus/defaults/preferences/tabmix.js
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