diff --git a/static/skin/viewer.js b/static/skin/viewer.js
index 2c3c80a14..a850b6693 100644
--- a/static/skin/viewer.js
+++ b/static/skin/viewer.js
@@ -204,14 +204,30 @@ function handle_visual_viewport_change() {
   contentIframe.height = wh - contentIframe.offsetTop - 4;
 }
 
+function setIframeUrl(path) {
+  try {
+    // Don't do anything if we are already at the requested URL.
+    // This is needed to break the infinite ping-pong played by
+    // handle_location_hash_change() and handle_content_url_change()
+    // (when either top-window or content window/iframe URL changes the other
+    // one is updated too).
+    if ( path == contentIframe.contentWindow.location.pathname )
+      return;
+  } catch ( error ) {
+    // This happens in Firefox when a PDF file is displayed in the iframe
+    // (sandboxing of the iframe content and cross-origin mismatch with the
+    // builtin PDF viewer result in preventing access to the attributes of
+    // contentIframe.contentWindow.location).
+    // Fall through and load the requested URL.
+  }
+  contentIframe.contentWindow.location.replace(path);
+}
+
 function handle_location_hash_change() {
   const hash = window.location.hash.slice(1);
   console.log("handle_location_hash_change: " + hash);
   updateCurrentBookIfNeeded(hash);
-  const iframeContentUrl = userUrl2IframeUrl(hash);
-  if ( iframeContentUrl != contentIframe.contentWindow.location.pathname ) {
-    contentIframe.contentWindow.location.replace(iframeContentUrl);
-  }
+  setIframeUrl(userUrl2IframeUrl(hash));
   updateSearchBoxForLocationChange();
   previousScrollTop = Infinity;
   history.replaceState(viewerState, null);
diff --git a/test/server.cpp b/test/server.cpp
index b5642719d..b48b488e2 100644
--- a/test/server.cpp
+++ b/test/server.cpp
@@ -73,7 +73,7 @@ const ResourceCollection resources200Compressible{
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/taskbar.css" },
   { STATIC_CONTENT,  "/ROOT%23%3F/skin/taskbar.css?cacheid=bbdaf425" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/viewer.js" },
-  { STATIC_CONTENT,  "/ROOT%23%3F/skin/viewer.js?cacheid=b9a574d4" },
+  { STATIC_CONTENT,  "/ROOT%23%3F/skin/viewer.js?cacheid=725c95a2" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/fonts/Poppins.ttf" },
   { STATIC_CONTENT,  "/ROOT%23%3F/skin/fonts/Poppins.ttf?cacheid=af705837" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/fonts/Roboto.ttf" },
@@ -312,7 +312,7 @@ R"EXPECTEDRESULT(    <link type="text/css" href="./skin/taskbar.css?cacheid=bbda
     <link type="text/css" href="./skin/css/autoComplete.css?cacheid=08951e06" rel="Stylesheet" />
     <script type="module" src="./skin/i18n.js?cacheid=2cf0f8c5" defer></script>
     <script type="text/javascript" src="./skin/languages.js?cacheid=648526e1" defer></script>
-    <script type="text/javascript" src="./skin/viewer.js?cacheid=b9a574d4" defer></script>
+    <script type="text/javascript" src="./skin/viewer.js?cacheid=725c95a2" defer></script>
     <script type="text/javascript" src="./skin/autoComplete.min.js?cacheid=1191aaaf"></script>
       const blankPageUrl = root + "/skin/blank.html?cacheid=6b1fa032";
             <img src="./skin/langSelector.svg?cacheid=00b59961">