Firefox preferences for all users are in /etc/firefox/syspref.js. You can copy settings from your current user preferences.
$ sudoedit /etc/firefox/syspref.js
Write default values pref() or forced values lockPref().
pref('browser.startup.homepage', "http://terokarvinen.com");
Close and open Firefox to see the effect.
If you (as the normal user) have already set the homepage (the initial page that opens on firefox load), this will override the global setting. In that case, you can test
lockPref('browser.startup.homepage', "http://google.com/ncr");
More values
To see all values you can set here, surf to
about:config
To see your current user settings, see
$ cat $HOME/.mozilla/firefox/*.default/prefs.js
This means that you can set these settings in Firefox user interface, choose your favorite settings in pref.js and copy these to system wide /etc/firefox/syspref.js.
Adminstrivia
Tested on Xubuntu 16.04 LTS amd64, Firefox 48.0+build2-0ubuntu0.16.04.1 installed with apt-get.