You can't import your prefs.js
as a module, because it isn't one.
What you can, and are encouraged to, do is import once the Services module and then reference the various services it offers through the global variable Services.
Edit:
Components.utils.import("resource://gre/modules/Services.jsm");
var prefs = Services.prefs;
var mypref = prefs.getBoolPref("extensions.myaddon.mypref");