diff --git a/chrome/content/zotero/xpcom/proxy.js b/chrome/content/zotero/xpcom/proxy.js
index 59020b084f..0d57a27c49 100644
--- a/chrome/content/zotero/xpcom/proxy.js
+++ b/chrome/content/zotero/xpcom/proxy.js
@@ -138,8 +138,13 @@ Zotero.Proxies = new function() {
 	 * @type String
 	 */
 	this.proxyToProper = function(url, onlyReturnIfProxied) {
-		 // make sure url has a trailing slash
-		url = new URL(url).href;
+		// make sure url has a trailing slash
+		if (typeof URL === 'undefined') {
+			url = new (Services.wm.getMostRecentWindow("navigator:browser")).URL(url).href;
+		}
+		else {
+			url = new URL(url).href;
+		}
 		for (let proxy of Zotero.Proxies.proxies) {
 			if(proxy.regexp) {
 				var m = proxy.regexp.exec(url);
@@ -179,10 +184,13 @@ Zotero.Proxies = new function() {
 	 * @returns {Object} Unproxied url to proxy object
 	 */
 	this.getPotentialProxies = function(url) {
-		try {
-			// make sure url has a trailing slash
+		// make sure url has a trailing slash
+		if (typeof URL === 'undefined') {
+			url = new (Services.wm.getMostRecentWindow("navigator:browser")).URL(url).href;
+		}
+		else {
 			url = new URL(url).href;
-		} catch (e) { }
+		}
 		var urlToProxy = {};
 		// If it's a known proxied URL just return it
 		if (Zotero.Proxies.transparent) {