In the process of improving localizability in Snowl today, I put together a StringBundle JS module to make it easier to access string bundles from other JS modules (and JS XPCOM components and chrome JS, for that matter).

The module has a simple, easy-to-use API with a single get method that retrieves both plain and formatted strings:

let strings = new StringBundle("chrome://example/locale/strings.properties");
let foo = strings.get("foo");
let barFormatted = strings.get("bar", [arg1, arg2]);
for each (let string in strings.getAll())
  dump(string.key + " = " + string.value + "n");

However, it also supports the API for the stringbundle XBL binding to make it easier to move from the binding to the module:

let strings = document.getElementById("myStringBundleElement");
              new StringBundle("chrome://example/locale/strings.properties");
let foo = strings.getString("foo");
let barFormatted = strings.getFormattedString("bar", [arg1, arg2]);
let enumerator = strings.strings;
while (enumerator.hasMoreElements()) {
  let string = enumerator.getNext().QueryInterface(Ci.nsIPropertyElement);
  dump(string.key + " = " + string.value + "n");
}

The module is available with the other useful JS modules in the jsmodules project on mozdev.org. Documentation is embedded inside the module as well as on the project wiki.