gBrowser.tabContainer.addEventListener('click', event => {
　if (overTabImage(event)) {
　　let tab = event.target;
　　gBrowser.removeTab(tab);
　}
}, true);
gBrowser.tabContainer.addEventListener('mousedown', event => {
　if (overTabImage(event)) event.stopPropagation();
}, true);
gBrowser.tabContainer.addEventListener('dragstart', event => {
　if (overTabImage(event)) event.stopPropagation();
}, true);

function overTabImage(event) {
　return (event.originalTarget.localName === 'image' &&
　　event.target.localName === 'tab' &&
　　!event.target.pinned);
}