diff --git a/src/js/contentscript-end.js b/src/js/contentscript-end.js
index 871d93d76..03e2b2a21 100644
--- a/src/js/contentscript-end.js
+++ b/src/js/contentscript-end.js
@@ -30,7 +30,7 @@
 
 if (vAPI.safari) {
     if (!/^https?:/.test(location.protocol)) {
-        throw "uBlock> contentscript-end.js > Skipping page: " + location.protocol + location.host;
+        throw "uBlock> contentscript-end.js > Skipping " + location.protocol;
     }
 }
 
@@ -418,21 +418,6 @@ var messager = vAPI.messaging.channel('contentscript-end.js');
         return;
     }
 
-    if (!window.MutationObserver) {
-        window.MutationObserver = window.WebKitMutationObserver || window.MozMutationObserver;
-
-        // dummy shim for older browsers
-        if (!window.MutationObserver) {
-            window.MutationObserver = function(handler) {
-                this.observe = function(target) {
-                    target.addEventListener('DOMNodeInserted', function(e) {
-                        handler([{addedNodes: [e.target]}]);
-                    }, true);
-                };
-            }
-        }
-    }
-
     var ignoreTags = {
         'link': true,
         'LINK': true,
diff --git a/src/js/vapi-client.js b/src/js/vapi-client.js
index b406713bc..69bd09c11 100644
--- a/src/js/vapi-client.js
+++ b/src/js/vapi-client.js
@@ -197,6 +197,20 @@ if (self.chrome) {
         return;
     }
 
+    window.MutationObserver = window.MutationObserver || window.WebKitMutationObserver;
+
+    if (!window.MutationObserver) {
+        // dummy, minimalistic shim for older versions (<6)
+        // only supports node insertions, but currently we don't use it for anything else
+        window.MutationObserver = function(handler) {
+            this.observe = function(target) {
+                target.addEventListener('DOMNodeInserted', function(e) {
+                    handler([{addedNodes: [e.target]}]);
+                }, true);
+            };
+        }
+    }
+
     var beforeLoadEvent = document.createEvent('Event');
     beforeLoadEvent.initEvent('beforeload');