--- a/toolkit/mozapps/extensions/content/extensions.js
+++ b/toolkit/mozapps/extensions/content/extensions.js
@@ -3467,16 +3467,20 @@ var gDetailView = {
         timeout = setTimeout(_resizeBrowser, TIMEOUT);
       }
     }
 
     function _resizeBrowser() {
       timeout = null;
 
       let doc = browser.contentDocument;
+      if (!doc) {
+        return;
+      }
+
       let body = doc.body || doc.documentElement;
 
       let docHeight = doc.documentElement.getBoundingClientRect().height;
 
       let height = Math.ceil(body.scrollHeight +
                              // Compensate for any offsets between the scroll
                              // area of the body and the outer height of the
                              // document.