/etc/firefox/syspref.js is in firefox 59.0.2+build1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | // This file can be used to configure global preferences for Firefox
// Example: Homepage
//pref("browser.startup.homepage", "http://www.weebls-stuff.com/wab/");
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