if(!aText) return false; //実行NG: falseを返す
if(exec){ //ドロップ
(function(){
Components.classes["@mozilla.org/widget/clipboardhelper;1"]
.getService(Components.interfaces.nsIClipboardHelper).copyString(aText);
})();

を

var word = this.getDropText(aDragSession);
if (!word) return false; //実行NG: falseを返す
if(exec){ //ドロップ
(function(){
Components.classes["@mozilla.org/widget/clipboardhelper;1"]
.getService(Components.interfaces.nsIClipboardHelper).copyString(word);
})();