--- a/mobile/chrome/content/browser.js
+++ b/mobile/chrome/content/browser.js
@@ -1017,17 +1017,17 @@ var Browser = {
 
     switch (aMessage.name) {
       case "Browser:ViewportMetadata":
         let tab = Browser.getTabForBrowser(browser);
         // Some browser such as iframes loaded dynamically into the chrome UI
         // does not have any assigned tab
         if (tab) {
           tab.updateViewportMetadata(json);
-          if (!this.isLoading())
+          if (!tab.isLoading())
             tab.updateThumbnail();
         }
         break;
 
       case "Browser:FormSubmit":
         browser.lastLocation = null;
         break;