Fix mozcentral build.

web/viewer.html

@@ -5,7 +5,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
     <title>PDF.js viewer</title>
 
-<!--#if FIREFOX-->
+<!--#if FIREFOX || MOZCENTRAL-->
 <!--#include viewer-snippet-firefox-extension.html-->
 <!--#endif-->
 
@@ -14,7 +14,7 @@
     <link rel="resource" type="application/l10n" href="locale.properties"/>
 <!--#endif-->
 
-<!--#if !FIREFOX -->
+<!--#if !(FIREFOX || MOZCENTRAL)-->
     <script type="text/javascript" src="compatibility.js"></script>
 <!--#endif-->