diff --git a/chrome/content/zotero/xpcom/http.js b/chrome/content/zotero/xpcom/http.js
index fec534de72..2ae32769f9 100644
--- a/chrome/content/zotero/xpcom/http.js
+++ b/chrome/content/zotero/xpcom/http.js
@@ -249,7 +249,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, onDone, responseCharset);
 		};
 		
@@ -337,7 +338,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, onDone, responseCharset);
 		};
 		
@@ -401,7 +403,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, onDone);
 		};
 		
@@ -440,7 +443,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, callback);
 		};
 		xmlhttp.send(null);
@@ -482,7 +486,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, function (xmlhttp) {
 				Zotero.debug("Proxy auth request completed with status "
 					+ xmlhttp.status + ": " + xmlhttp.responseText);
@@ -561,7 +566,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, callback);
 		};
 		
@@ -597,7 +603,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, callback);
 		};
 		xmlhttp.send(null);
@@ -642,7 +649,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, callback);
 		};
 		xmlhttp.send(body);
@@ -681,7 +689,8 @@ Zotero.HTTP = new function() {
 		var useMethodjit = Components.utils.methodjit;
 		/** @ignore */
 		xmlhttp.onreadystatechange = function() {
-			Components.utils.methodjit = useMethodjit;
+			// XXX Remove when we drop support for Fx <24
+			if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 			_stateChange(xmlhttp, callback);
 		};
 		xmlhttp.send(null);
diff --git a/chrome/content/zotero/xpcom/zotero.js b/chrome/content/zotero/xpcom/zotero.js
index 70c786e179..4497229dd1 100644
--- a/chrome/content/zotero/xpcom/zotero.js
+++ b/chrome/content/zotero/xpcom/zotero.js
@@ -1545,7 +1545,8 @@ Components.utils.import("resource://gre/modules/Services.jsm");
 			yielded,
 			useJIT = Components.utils.methodjit;
 		var timerCallback = {"notify":function() {
-			Components.utils.methodjit = useJIT;
+			// XXX Remove when we drop support for Fx <24
+			if(useJIT !== undefined) Components.utils.methodjit = useJIT;
 			
 			var err = false;
 			_waiting--;
@@ -1610,7 +1611,8 @@ Components.utils.import("resource://gre/modules/Services.jsm");
 			createInstance(Components.interfaces.nsITimer),
 			useJIT = Components.utils.methodjit;
 		var timerCallback = {"notify":function() {
-			Components.utils.methodjit = useJIT;
+			// XXX Remove when we drop support for Fx <24
+			if(useJIT !== undefined) Components.utils.methodjit = useJIT;
 			
 			if(_waiting && !runWhenWaiting) {
 				// if our callback gets called during Zotero.wait(), queue it to be set again
diff --git a/resource/q.js b/resource/q.js
index 029b5beddc..f73a913369 100644
--- a/resource/q.js
+++ b/resource/q.js
@@ -70,7 +70,8 @@
 					timer = Components.classes["@mozilla.org/timer;1"].
 						createInstance(Components.interfaces.nsITimer);
 				timer.initWithCallback({"notify":function() {
-					Components.utils.methodjit = useMethodjit;
+                    // XXX Remove when we drop support for Fx <24
+					if(useMethodjit !== undefined) Components.utils.methodjit = useMethodjit;
 					
 					// Remove timer from array so it can be garbage collected
 					_runningTimers.splice(_runningTimers.indexOf(timer), 1);