diff --git a/content/base/src/nsGkAtomList.h b/content/base/src/nsGkAtomList.h
index 2cd34edd43e6..b68ffab5b378 100755
--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -514,6 +514,7 @@ GK_ATOM(mouseout, "mouseout")
 GK_ATOM(mouseover, "mouseover")
 GK_ATOM(mousethrough, "mousethrough")
 GK_ATOM(mouseup, "mouseup")
+GK_ATOM(moz_opaque, "moz-opaque")
 GK_ATOM(msthemecompatible, "msthemecompatible")
 GK_ATOM(multicol, "multicol")
 GK_ATOM(multiple, "multiple")
diff --git a/content/canvas/public/nsICanvasRenderingContextInternal.h b/content/canvas/public/nsICanvasRenderingContextInternal.h
index fc14f06ea778..6c4e27b71f09 100644
--- a/content/canvas/public/nsICanvasRenderingContextInternal.h
+++ b/content/canvas/public/nsICanvasRenderingContextInternal.h
@@ -77,6 +77,12 @@ public:
   // If this canvas context can be represented with a simple Thebes surface,
   // return the surface.  Otherwise returns an error.
   NS_IMETHOD GetThebesSurface(gfxASurface **surface) = 0;
+
+  // If this context is opaque, the backing store of the canvas should
+  // be created as opaque; all compositing operators should assume the
+  // dst alpha is always 1.0.  If this is never called, the context
+  // defaults to false (not opaque).
+  NS_IMETHOD SetIsOpaque(PRBool isOpaque) = 0;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsICanvasRenderingContextInternal,
diff --git a/content/canvas/src/nsCanvasRenderingContext2D.cpp b/content/canvas/src/nsCanvasRenderingContext2D.cpp
index 33c38b1005be..84d4807a5383 100644
--- a/content/canvas/src/nsCanvasRenderingContext2D.cpp
+++ b/content/canvas/src/nsCanvasRenderingContext2D.cpp
@@ -296,6 +296,7 @@ public:
                               const PRUnichar* aEncoderOptions,
                               nsIInputStream **aStream);
     NS_IMETHOD GetThebesSurface(gfxASurface **surface);
+    NS_IMETHOD SetIsOpaque(PRBool isOpaque);
 
     // nsISupports interface
     NS_DECL_ISUPPORTS
@@ -334,7 +335,8 @@ protected:
 
     // Member vars
     PRInt32 mWidth, mHeight;
-    PRBool mValid;
+    PRPackedBool mValid;
+    PRPackedBool mOpaque;
 
     // the canvas element informs us when it's going away,
     // so these are not nsCOMPtrs
@@ -470,7 +472,7 @@ NS_NewCanvasRenderingContext2D(nsIDOMCanvasRenderingContext2D** aResult)
 }
 
 nsCanvasRenderingContext2D::nsCanvasRenderingContext2D()
-    : mValid(PR_FALSE), mCanvasElement(nsnull),
+    : mValid(PR_FALSE), mOpaque(PR_FALSE), mCanvasElement(nsnull),
       mSaveCount(0), mCairo(nsnull), mSurface(nsnull), mStyleStack(20)
 {
 }
@@ -685,7 +687,12 @@ nsCanvasRenderingContext2D::SetDimensions(PRInt32 width, PRInt32 height)
 
     // Check that the dimensions are sane
     if (gfxASurface::CheckSurfaceSize(gfxIntSize(width, height), 0xffff)) {
-        mThebesSurface = gfxPlatform::GetPlatform()->CreateOffscreenSurface(gfxIntSize(width, height), gfxASurface::ImageFormatARGB32);
+        gfxASurface::gfxImageFormat format = gfxASurface::ImageFormatARGB32;
+        if (mOpaque)
+            format = gfxASurface::ImageFormatRGB24;
+
+        mThebesSurface = gfxPlatform::GetPlatform()->CreateOffscreenSurface
+            (gfxIntSize(width, height), format);
 
         if (mThebesSurface->CairoStatus() == 0) {
             mThebesContext = new gfxContext(mThebesSurface);
@@ -732,6 +739,24 @@ nsCanvasRenderingContext2D::SetDimensions(PRInt32 width, PRInt32 height)
 
     return NS_OK;
 }
+
+NS_IMETHODIMP
+nsCanvasRenderingContext2D::SetIsOpaque(PRBool isOpaque)
+{
+    if (isOpaque == mOpaque)
+        return NS_OK;
+
+    mOpaque = isOpaque;
+
+    if (mValid) {
+        /* If we've already been created, let SetDimensions take care of
+         * recreating our surface
+         */
+        return SetDimensions(mWidth, mHeight);
+    }
+
+    return NS_OK;
+}
  
 NS_IMETHODIMP
 nsCanvasRenderingContext2D::Render(gfxContext *ctx)
@@ -748,12 +773,19 @@ nsCanvasRenderingContext2D::Render(gfxContext *ctx)
 
     nsRefPtr<gfxPattern> pat = new gfxPattern(mThebesSurface);
 
+    gfxContext::GraphicsOperator op = ctx->CurrentOperator();
+    if (mOpaque)
+        ctx->SetOperator(gfxContext::OPERATOR_SOURCE);
+
     // XXX I don't want to use PixelSnapped here, but layout doesn't guarantee
     // pixel alignment for this stuff!
     ctx->NewPath();
     ctx->PixelSnappedRectangleAndSetPattern(gfxRect(0, 0, mWidth, mHeight), pat);
     ctx->Fill();
 
+    if (mOpaque)
+        ctx->SetOperator(op);
+
     return rv;
 }
 
diff --git a/content/html/content/src/nsHTMLCanvasElement.cpp b/content/html/content/src/nsHTMLCanvasElement.cpp
index 692f4281b01d..005275d5edac 100644
--- a/content/html/content/src/nsHTMLCanvasElement.cpp
+++ b/content/html/content/src/nsHTMLCanvasElement.cpp
@@ -118,6 +118,8 @@ public:
 
 protected:
   nsIntSize GetWidthHeight();
+  PRBool GetIsOpaque();
+
   nsresult UpdateContext();
   nsresult ToDataURLImpl(const nsAString& aMimeType,
                          const nsAString& aEncoderOptions,
@@ -191,8 +193,15 @@ nsHTMLCanvasElement::GetWidthHeight()
   return size;
 }
 
+PRBool
+nsHTMLCanvasElement::GetIsOpaque()
+{
+  return HasAttr(kNameSpaceID_None, nsGkAtoms::moz_opaque);
+}
+
 NS_IMPL_INT_ATTR_DEFAULT_VALUE(nsHTMLCanvasElement, Width, width, DEFAULT_CANVAS_WIDTH)
 NS_IMPL_INT_ATTR_DEFAULT_VALUE(nsHTMLCanvasElement, Height, height, DEFAULT_CANVAS_HEIGHT)
+NS_IMPL_BOOL_ATTR(nsHTMLCanvasElement, MozOpaque, moz_opaque)
 
 nsresult
 nsHTMLCanvasElement::SetAttr(PRInt32 aNameSpaceID, nsIAtom* aName,
@@ -202,7 +211,7 @@ nsHTMLCanvasElement::SetAttr(PRInt32 aNameSpaceID, nsIAtom* aName,
   nsresult rv = nsGenericHTMLElement::SetAttr(aNameSpaceID, aName, aPrefix, aValue,
                                               aNotify);
   if (NS_SUCCEEDED(rv) && mCurrentContext &&
-      (aName == nsGkAtoms::width || aName == nsGkAtoms::height))
+      (aName == nsGkAtoms::width || aName == nsGkAtoms::height || aName == nsGkAtoms::moz_opaque))
   {
     rv = UpdateContext();
     NS_ENSURE_SUCCESS(rv, rv);
@@ -221,6 +230,9 @@ nsHTMLCanvasElement::GetAttributeChangeHint(const nsIAtom* aAttribute,
       aAttribute == nsGkAtoms::height)
   {
     NS_UpdateHint(retval, NS_STYLE_HINT_REFLOW);
+  } else if (aAttribute == nsGkAtoms::moz_opaque)
+  {
+    NS_UpdateHint(retval, NS_STYLE_HINT_VISUAL);
   }
   return retval;
 }
@@ -488,6 +500,7 @@ nsHTMLCanvasElement::UpdateContext()
   nsresult rv = NS_OK;
   if (mCurrentContext) {
     nsIntSize sz = GetWidthHeight();
+    rv = mCurrentContext->SetIsOpaque(GetIsOpaque());
     rv = mCurrentContext->SetDimensions(sz.width, sz.height);
   }
 
diff --git a/dom/public/idl/html/nsIDOMHTMLCanvasElement.idl b/dom/public/idl/html/nsIDOMHTMLCanvasElement.idl
index 2cd712419bc4..e3207497162b 100644
--- a/dom/public/idl/html/nsIDOMHTMLCanvasElement.idl
+++ b/dom/public/idl/html/nsIDOMHTMLCanvasElement.idl
@@ -52,6 +52,7 @@ interface nsIDOMHTMLCanvasElement : nsIDOMHTMLElement
 {
   attribute long width;
   attribute long height;
+  attribute boolean mozOpaque;
 
   nsISupports getContext(in DOMString contextId);