diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index d5965dc6f700..cab74daa2d91 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -3946,9 +3946,8 @@ nsDocumentViewer::PrintPreview(nsIPrintSettings* aPrintSettings,
                                nsIWebProgressListener* aWebProgressListener)
 {
 #if defined(NS_PRINTING) && defined(NS_PRINT_PREVIEW)
-  NS_WARNING_ASSERTION(
-    IsInitializedForPrintPreview(),
-    "Using docshell.printPreview is the preferred way for print previewing!");
+  MOZ_ASSERT(IsInitializedForPrintPreview(),
+    "For print preview nsIWebBrowserPrint must be from docshell.printPreview!");
 
   NS_ENSURE_ARG_POINTER(aChildDOMWin);
   nsresult rv = NS_OK;
diff --git a/layout/base/tests/chrome/printpreview_bug396024_helper.xul b/layout/base/tests/chrome/printpreview_bug396024_helper.xul
index 262bb3469e2a..ce1d6179f2e3 100644
--- a/layout/base/tests/chrome/printpreview_bug396024_helper.xul
+++ b/layout/base/tests/chrome/printpreview_bug396024_helper.xul
@@ -17,8 +17,7 @@ var todo = window.opener.wrappedJSObject.todo;
 var SimpleTest = window.opener.wrappedJSObject.SimpleTest;
 var gWbp;
 function printpreview() {
-  gWbp = window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-             .getInterface(Components.interfaces.nsIWebBrowserPrint);
+  gWbp = window.frames[1].document.docShell.printPreview;
   var listener = {
     onLocationChange: function(webProgress, request, location, flags) { },
     onProgressChange: function(webProgress, request, curSelfProgress, 
@@ -44,8 +43,7 @@ function printpreview() {
 }
 
 function exitprintpreview() {
-  window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-   .getInterface(Components.interfaces.nsIWebBrowserPrint).exitPrintPreview(); 
+  window.frames[1].document.docShell.printPreview.exitPrintPreview();
 }
 
 function finish() {
@@ -91,8 +89,7 @@ function run2() {
 }
 
 function run3() {
-  gWbp = window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-             .getInterface(Components.interfaces.nsIWebBrowserPrint);
+  gWbp = window.frames[1].document.docShell.printPreview;
   ok(gWbp.doingPrintPreview, "Should be doing print preview");
   exitprintpreview();
   setTimeout(run4, 0);
@@ -108,12 +105,12 @@ function run4() {
   i.addEventListener("load", loadhandler, true);
   document.documentElement.getBoundingClientRect();
   document.documentElement.appendChild(i);
-  gWbp = window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-             .getInterface(Components.interfaces.nsIWebBrowserPrint);
-  ok(!gWbp.doingPrintPreview, "Should not be doing print preview anymore2");
 }
 
 function run5() {
+  gWbp = window.frames[0].document.docShell.printPreview;
+  ok(!gWbp.doingPrintPreview, "Should not be doing print preview anymore2");
+
   //XXX this shouldn't be necessary, see bug 405555
   printpreview();
   exitprintpreview();
diff --git a/layout/base/tests/chrome/printpreview_bug482976_helper.xul b/layout/base/tests/chrome/printpreview_bug482976_helper.xul
index 34c2d658ec74..9a64888d5c2f 100644
--- a/layout/base/tests/chrome/printpreview_bug482976_helper.xul
+++ b/layout/base/tests/chrome/printpreview_bug482976_helper.xul
@@ -17,8 +17,7 @@ var todo = window.opener.wrappedJSObject.todo;
 var SimpleTest = window.opener.wrappedJSObject.SimpleTest;
 var gWbp;
 function printpreview() {
-  gWbp = window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-             .getInterface(Components.interfaces.nsIWebBrowserPrint);
+  gWbp = window.frames[1].document.docShell.printPreview;
   var listener = {
     onLocationChange: function(webProgress, request, location, flags) { },
     onProgressChange: function(webProgress, request, curSelfProgress, 
@@ -44,8 +43,7 @@ function printpreview() {
 }
 
 function exitprintpreview() {
-  window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-   .getInterface(Components.interfaces.nsIWebBrowserPrint).exitPrintPreview(); 
+  window.frames[1].document.docShell.printPreview.exitPrintPreview();
 }
 
 function finish() {
diff --git a/layout/base/tests/chrome/printpreview_helper.xul b/layout/base/tests/chrome/printpreview_helper.xul
index d0bef8fb047c..72b24530970c 100644
--- a/layout/base/tests/chrome/printpreview_helper.xul
+++ b/layout/base/tests/chrome/printpreview_helper.xul
@@ -24,8 +24,7 @@ var file = Components.classes["@mozilla.org/file/directory_service;1"]
 filePath = file.path;
 
 function printpreview() {
-  gWbp = window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-             .getInterface(Components.interfaces.nsIWebBrowserPrint);
+  gWbp = window.frames[1].document.docShell.printPreview;
   var listener = {
     onLocationChange: function(webProgress, request, location, flags) { },
     onProgressChange: function(webProgress, request, curSelfProgress, 
@@ -61,8 +60,7 @@ function printpreview() {
 }
 
 function exitprintpreview() {
-  window.frames[1].QueryInterface(Components.interfaces.nsIInterfaceRequestor)
-   .getInterface(Components.interfaces.nsIWebBrowserPrint).exitPrintPreview(); 
+  window.frames[1].document.docShell.printPreview.exitPrintPreview();
 }
 
 function finish() {