fix viewing of pdfbook on firefox

lms/templates/static_pdfbook.html

@@ -90,9 +90,8 @@
           </div>
         </div>
 
-%if 'chapters' in textbook:
+        %if 'chapters' in textbook:
 	<section aria-label="Textbook Navigation" class="book-sidebar">
-
 	  <ul id="booknav" class="treeview-booknav">
 	      <%def name="print_entry(entry, index_value)">
 		  <li id="pdfchapter-${index_value}">
@@ -107,9 +106,10 @@
 	      % endfor
 	  </ul>
 	</section>
-%endif
+        %endif
 
         <section id="viewerContainer" class="book">
+	  <section class="page">
 	    <!-- use same page-turning as used in image-based textbooks -->
             <nav>
               <ul>
@@ -121,10 +121,9 @@
                 </li>
               </ul>
             </nav>
-
             <div id="viewer" contextmenu="viewerContextMenu"></div>
-        </div>
-
+          </section>
+        </section>
 
       </div> <!-- mainContainer -->