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Obsolete since Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Support for extensions using XUL/XPCOM or the Add-on SDK was removed in Firefox 57, released November 2017. As there is no supported version of Firefox enabling these technologies, this page will be removed by December 2020.

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

Starting from Firefox 53, no new legacy add-ons will be accepted on (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.


Handling onclosetab Event

Assuming the tabbox, tabs, and tabpanels widgets with id's the same as their nodeName, this function will correctly remove the current tab and tab panel for the onclosetab tabs event:

function removeTab(){
    var tabbox = document.getElementById("tabbox");
    var currentIndex = tabbox.selectedIndex;
    var tabs=document.getElementById("tabs");
    var tabpanels=document.getElementById("tabpanels");
    /*Work around If last tab is removed, widget fails to advance to next tab*/
    if(-1 == tabbox.selectedIndex && tabs.childNodes.length>0){

Creating a close tab button

To have a tab close button, you must configure the style.


   .tabs-closebutton {
     list-style-image: url(;
   .tabs-closebutton {
     list-style-image: url("chrome://global/skin/icons/Close.gif");