function getExtensionPrefs(prefix) {
        return PreferencesImpl.manager.getPrefixedSystem(prefix);
    }
Creates an extension-specific preferences manager using the prefix given.
A `.` character will be appended to the prefix. So, a preference named `foo`
with a prefix of `myExtension` will be stored as `myExtension.foo` in the
preferences files.

@param {string} prefix Prefix to be applied