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.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
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 WebExtensions if they can. See the "Compatibility Milestones" document for more.
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.
function lookupPrefix (node, namespaceURI) {
var htmlMode = document.contentType; // Mozilla only
// Depends on private function _lookupNamespacePrefix() below and on https://developer.mozilla.org/En/Code_snippets/LookupNamespaceURI
// http://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-lookupNamespacePrefix
// http://www.w3.org/TR/DOM-Level-3-Core/namespaces-algorithms.html#lookupNamespacePrefixAlgo
// (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 11: // Node.DOCUMENT_FRAGMENT_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;
default:
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 || att.name.substr(att.name.indexOf(':')+1); // latter test for IE which doesn't support localName
if (localName.indexOf(':') !== -1) { // For Firefox when in HTML mode
localName = localName.substr(att.name.indexOf(':')+1);
}
if (
xmlnsPattern.test(att.name) &&
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;
}