MDN will switch to a new design later today, a sneak peek: https://blog.mozilla.org/opendesign/mdns-new-design-beta/

Add New Tab

var aURL = "chrome://myext/mytab.xul";
let tabmail = getMail3Pane().document.getElementById("tabmail");
tabmail.openTab("chromeTab", { chromePage: aUrl });

Document Tags and Contributors

 Last updated by: wbamberg,