如何从Firefox加载项调用sessionStorage?

我正在打电话 sessionStorage 在我的插件中(这是演示 https://builder.addons.mozilla.org/package/154290/latest/ ),但什么都不会存储。有什么建议吗?

 var menuItem = contextMenu.Item({
    label: "Check with Proofread Bot",
    context: contextMenu.SelectionContext(),
    contentScript: 'self.on("click", function () {' +
               '  var text = window.getSelection().toString();' +
                '  sessionStorage.setItem("proofread_bot_chrome", text);' +
                '  self.postMessage(text);' +
                        '});',
    onMessage: function(text) {
        tabs.open("http://proofreadbot.com");                    }                        
    }) ;