diff --git a/web/viewer.css b/web/viewer.css
index e355f7fc2..b9fd3e9e4 100644
--- a/web/viewer.css
+++ b/web/viewer.css
@@ -9,7 +9,7 @@ body {
 }
 
 [hidden] {
-  display: none;
+  display: none !important;
 }
 
 /* === Toolbar === */
diff --git a/web/viewer.html b/web/viewer.html
index f395292f9..ad9d1189e 100644
--- a/web/viewer.html
+++ b/web/viewer.html
@@ -92,7 +92,7 @@
 
       <input id="fileInput" type="file" oncontextmenu="return false;"/>
 
-      <div class="separator"></div>
+      <div id="fileInputSeperator" class="separator"></div>
 
       <a href="#" id="viewBookmark" title="Bookmark (or copy) current location">
         <img src="images/bookmark.svg" alt="Bookmark" align="top" height="16"/>
diff --git a/web/viewer.js b/web/viewer.js
index 1ed9019ff..131d12b42 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -995,10 +995,14 @@ window.addEventListener('load', function webViewerLoad(evt) {
               window.location.toString() : params.file || kDefaultURL;
   PDFView.open(file, parseFloat(scale));
 
-  if (!window.File || !window.FileReader || !window.FileList || !window.Blob)
+  if (PDFJS.isFirefoxExtension || !window.File || !window.FileReader ||
+      !window.FileList || !window.Blob) {
     document.getElementById('fileInput').setAttribute('hidden', 'true');
-  else
+    document.getElementById('fileInputSeperator')
+                              .setAttribute('hidden', 'true');
+  } else {
     document.getElementById('fileInput').value = null;
+  }
 
   if ('disableWorker' in params)
     PDFJS.disableWorker = (params['disableWorker'] === 'true');