>>746
clickをmousedownにしたら動いた
(function () {
gBrowser.mTabContainer.addEventListener('mousedown', function (event) {
var tab = document.evaluate('ancestor-or-self::*[local-name()="tab"][1]', event.originalTarget, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
if (event.button === 0 && event.originalTarget.className === 'tab-icon-image') {
gBrowser.removeTab(tab);
}
}, false);
}());