ご参考に
https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox/Frame_script_environment

// Google検索
var mm = gBrowser.selectedBrowser.messageManager;
mm.addMessageListener('getSelection', function listener(message) {
var t = (message.data !== '') ? message.data : window.prompt('Google:', '');
if (t) {
if ('TreeStyleTabService' in window) TreeStyleTabService.readyToOpenChildTab(gBrowser.selectedTab);
gBrowser.addTab('https://www.google.co.jp/search?q=' + encodeURI(t));
}
mm.removeMessageListener('getSelection', listener, true);
});
mm.loadFrameScript('data:,sendAsyncMessage("getSelection", content.document.getSelection ? content.document.getSelection().toString() : "")', true);

// 訪問済みリンクを灰色に
var mm = gBrowser.selectedBrowser.messageManager;
mm.loadFrameScript('data:,' + encodeURIComponent((function (css) {
var head = content.document.getElementsByTagName('head') [0];
if (head) {
var style = content.document.createElement('style');
style.type = 'text/css';
style.innerHTML = css;
head.appendChild(style);
}
}).toSource() + '("a:visited,a:visited *{color:#ccc !important;}");'), true);