Firefox preferences: How to override defaults?

I want to override (but not lock) Firefox preferences for all users of a given host, including but not limited to the homepage. Here's something that doesn't work:

# /etc/firefox/pref/foobar.js
pref('browser.startup.homepage', "http://askubuntu.com");

Where is the authoritative documentation for firefox configuration management? Where are the text files that comprise all preferences and their default values? Which paths are searched and in what order?