Teste mal:
JavaScript
(function() {
if (location != 'chrome://browser/content/browser.xhtml')
return;
let menuitem = document.createXULElement('menuitem');
menuitem.id = 'context_closeLeftTabs';
menuitem.setAttribute('label', 'Linke Tabs schließen');
menuitem.setAttribute('oncommand',
'for (let i = TabContextMenu.contextTab._tPos - 1; i >= 0; i--) \
gBrowser.removeTab(gBrowser.tabs[i], {animate: true})');
let tabContextMenu = document.getElementById('tabContextMenu');
let refItem = document.getElementById('context_undoCloseTab');
tabContextMenu.insertBefore(menuitem, refItem);
tabContextMenu.insertBefore(document.getElementById('context_closeTabsToTheEnd'), refItem);
tabContextMenu.insertBefore(document.getElementById('context_closeOtherTabs'), refItem);
tabContextMenu.removeChild(document.getElementById('context_closeTabOptions'));
})();
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