For XPI installation notifications, there needs to be a frozen interface set for nsIXPIListener and nsISoftwareUpdate, or their equivalents. This will allow XPI install to be used with other install operations, allowing for one UI to be available.
Just to clarify, the frozen interfaces are needed to allow any component to controll the xpi install experience. There are cases where a xpi package must be installed and another browser component needs to be notified of the completion and status.
The xpinstall script engine has been removed from the trunk, bugs in it are obsolete.