The extra wrapper is for Firefox 17.
Without that Fx 17 drops me a line:

    Error: 'JavaScript component does not have a method named: "done"' when calling method: [nsIFilePickerShownCallback::done] = NS_ERROR_XPC_JSOBJECT_HAS_NO_FUNCTION_NAMED

The change that allow us to drop the `done` property was landed by [Bug 785744][1] in Firefox 18 and wasn't backported to 17.

Should I morph `doneCallback` into an object which implements `nsIFilePickerShownCallback`?
I happily do that change.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=781973 "Bug 781973 - Use filepicker's open() instead of the obsolete show() in /browser/*"