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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.


Here is an implementation of lookupPrefix which should work cross-browser.

Note that all Gecko-based browsers (including Firefox) support Node.lookupPrefix. This function is not necessary for Gecko-based browsers when used in XHTML.
function lookupPrefix (node, namespaceURI) {
	var htmlMode = document.contentType; // Mozilla only
	// Depends on private function _lookupNamespacePrefix() below and on
	// (The above had a few apparent 'bugs' in the pseudo-code which were corrected here)
	if (node.lookupPrefix && htmlMode !== 'text/html') { // Shouldn't use this in text/html for Mozilla as will return null
		  return node.lookupPrefix(namespaceURI);
	if (namespaceURI === null || namespaceURI === '') {
		return null;
	switch (node.nodeType) {
		case 1: // Node.ELEMENT_NODE
			return _lookupNamespacePrefix(namespaceURI, node);
		case 9: // Node.DOCUMENT_NODE
			return _lookupNamespacePrefix(namespaceURI, node.documentElement);
		case 6: // Node.ENTITY_NODE
		case 12: // Node.NOTATION_NODE
		case 10: // Node.DOCUMENT_TYPE_NODE
			return null;  // type is unknown
		case 2: // Node.ATTRIBUTE_NODE
			if (node.ownerElement) {
				return _lookupNamespacePrefix(namespaceURI, node.ownerElement);
			return null;
			if (node.parentNode) {
				// EntityReferences may have to be skipped to get to it
				return _lookupNamespacePrefix(namespaceURI, node.parentNode);
			return null;

// Private function for lookupPrefix only
function _lookupNamespacePrefix (namespaceURI, originalElement) {
	var xmlnsPattern = /^xmlns:(.*)$/;
	if (originalElement.namespaceURI && originalElement.namespaceURI === namespaceURI &&
			originalElement.prefix && originalElement.lookupNamespaceURI(originalElement.prefix) === namespaceURI) {
		return originalElement.prefix;
	if (originalElement.attributes && originalElement.attributes.length) {
		for (var i=0; i < originalElement.attributes.length; i++) {
			var att = originalElement.attributes[i];
			xmlnsPattern.lastIndex = 0;
			var localName = att.localName ||':')+1); // latter test for IE which doesn't support localName
			if (localName.indexOf(':') !== -1) { // For Firefox when in HTML mode
				localName = localName.substr(':')+1);
			if (
				xmlnsPattern.test( &&
				att.value === namespaceURI &&
				lookupNamespaceURI(originalElement, localName) === namespaceURI
			) {
				return localName;
	if (originalElement.parentNode) {
		// EntityReferences may have to be skipped to get to it
		return _lookupNamespacePrefix(namespaceURI, originalElement.parentNode);
	return null;