init : function() {
        var target = this.target;
        this.types.forEach(function(aType) {
            target.events.addListener(aType, target); // ←ココ
        });
    },

for (var i in this.types) {
  target.events.addListener(this.types[i], this);
}

function FUELEventListener(aTypes, aTarget, aHandler) {
    this.types = aTypes.split(/,\s*|\s+/);
    this.target = aTarget;
    this.handleEvent = aHandler;
}
FUELEventListener.prototype = {
    init : function() {
        this.types.forEach(function(aType) {
            this.target.events.addListener(aType, this);
        }, this);
    },
    destroy : function() {
        this.types.forEach(function(aType) {
            this.target.events.removeListener(aType, this);
        }, this);
        this.types = this.target = this.handleEvent = null;
    }
};
var pref = Application.prefs.get('browser.sessionstore.resume_session_once');
var listener = new FUELEventListener('change', pref,
      function(aEventItem) {
        alert('pref is changed!');
        this.destroy();
      }
    );
listener.init();