diff --git a/common/modules/buffer.jsm b/common/modules/buffer.jsm
index d41b703c..eba6158b 100644
--- a/common/modules/buffer.jsm
+++ b/common/modules/buffer.jsm
@@ -273,7 +273,7 @@ var Buffer = Module("Buffer", {
     get pageHeight() this.win.innerHeight,
 
     get contentViewer() this.docShell.contentViewer
-                                     .QueryInterface(Components.interfaces.nsIMarkupDocumentViewer),
+                                     .QueryInterface(Ci.nsIMarkupDocumentViewer || Ci.nsIContentViewer),
 
     /**
      * @property {number} The current browser's zoom level, as a