/usr/share/xul-ext/ubufox/defaults/preferences/ubuntu-gnome-homepage.properties is in ubuntu-gnome-default-settings 14.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # to test, just enable the example homepage pref in /etc/xul-ext/ubufox.js
browser.startup.homepage=about:home
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