In my firefox extension onUninstalled event doesn't seem to fire

Firefox 4 provides some events to detect when a user uninstalls an extension. I can get onUninstallingto start (when the user clicks to remove the extension), but I can’t do anything when it works onUninstalled(what should happen after the restart).

Components.utils.import("resource://gre/modules/AddonManager.jsm");
AddonManager.addAddonListener({
    onUninstalled: function(addon){
        //does not work
        alert("uninstalled!");
    },
    onUninstalling: function(addon){
        //works as expected
        alert("uninstalling!");   
    }
  });

Ultimately, I hope to show the page after the user disables my extension. I would rather do it after onUninstalled, if I could.