/usr/share/xul-ext/ is in xul-ext-ubufox 2.8-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/xul-ext/ubufox/
/usr/share/xul-ext/ubufox/chrome/
/usr/share/xul-ext/ubufox/chrome.manifest
/usr/share/xul-ext/ubufox/chrome/ubufox.jar
/usr/share/xul-ext/ubufox/components/
/usr/share/xul-ext/ubufox/components/AboutStartpage.js
/usr/share/xul-ext/ubufox/components/UpdateNotifier.js
/usr/share/xul-ext/ubufox/defaults/
/usr/share/xul-ext/ubufox/defaults/preferences/
/usr/share/xul-ext/ubufox/defaults/preferences/000system.js
/usr/share/xul-ext/ubufox/defaults/preferences/ubufox.js
/usr/share/xul-ext/ubufox/defaults/preferences/ubuntu-mods.js
/usr/share/xul-ext/ubufox/install.rdf
/usr/share/xul-ext/ubufox/scripts/
/usr/share/xul-ext/ubufox/scripts/check-component-enabled.py