Open Tools > Error Console  and paste the following

Components.classes['@mozilla.org/appshell/window-mediator;1'].getService(Components.interfaces.nsIWindowMediator).getMostRecentWindow("mail:3pane").document.getElementById("tabmail").openTab("contentTab", {contentPage: "[mail.google.com]"});

discovered by way of Lifehacker, reproduced here so I have a memorable place to copy/paste from the next time I close the tab by accident.

Dear lazyweb: how can this be turned into a button, menu item, or preference checkbox?