--- a/modules/libpr0n/src/SVGDocumentWrapper.cpp
+++ b/modules/libpr0n/src/SVGDocumentWrapper.cpp
@@ -63,35 +63,40 @@
 
 using namespace mozilla::dom;
 
 namespace mozilla {
 namespace imagelib {
 
 nsIAtom* SVGDocumentWrapper::kSVGAtom = nsnull; // lazily initialized
 
-NS_IMPL_ISUPPORTS3(SVGDocumentWrapper,
+NS_IMPL_ISUPPORTS4(SVGDocumentWrapper,
                    nsIStreamListener,
                    nsIRequestObserver,
-                   nsIObserver)
+                   nsIObserver,
+                   nsISupportsWeakReference)
 
 SVGDocumentWrapper::SVGDocumentWrapper()
- : mIgnoreInvalidation(PR_FALSE)
+  : mIgnoreInvalidation(PR_FALSE),
+    mRegisteredForXPCOMShutdown(PR_FALSE)
 {
   // Lazy-initialize our "svg" atom.  (It'd be nicer to just use nsGkAtoms::svg
   // directly, but we can't access it from here in non-libxul builds.)
   if (!SVGDocumentWrapper::kSVGAtom) {
     SVGDocumentWrapper::kSVGAtom =
       NS_NewPermanentAtom(NS_LITERAL_STRING("svg"));
   }
 }
 
 SVGDocumentWrapper::~SVGDocumentWrapper()
 {
   DestroyViewer();
+  if (mRegisteredForXPCOMShutdown) {
+    UnregisterForXPCOMShutdown();
+  }
 }
 
 void
 SVGDocumentWrapper::DestroyViewer()
 {
   if (mViewer) {
     mViewer->GetDocument()->OnPageHide(PR_FALSE, nsnull);
     mViewer->Close(nsnull);
@@ -371,26 +376,46 @@ SVGDocumentWrapper::SetupViewer(nsIReque
 
   RegisterForXPCOMShutdown();
   return NS_OK;
 }
 
 void
 SVGDocumentWrapper::RegisterForXPCOMShutdown()
 {
+  NS_ABORT_IF_FALSE(!mRegisteredForXPCOMShutdown,
+                    "re-registering for XPCOM shutdown");
   // Listen for xpcom-shutdown so that we can drop references to our
   // helper-document at that point. (Otherwise, we won't get cleaned up
   // until imgLoader::Shutdown, which can happen after the JAR service
   // and RDF service have been unregistered.)
   nsresult rv;
   nsCOMPtr<nsIObserverService> obsSvc = do_GetService(OBSERVER_SVC_CID, &rv);
   if (NS_FAILED(rv) ||
       NS_FAILED(obsSvc->AddObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID,
-                                    PR_FALSE))) {
+                                    PR_TRUE))) {
     NS_WARNING("Failed to register as observer of XPCOM shutdown");
+  } else {
+    mRegisteredForXPCOMShutdown = PR_TRUE;
+  }
+}
+
+void
+SVGDocumentWrapper::UnregisterForXPCOMShutdown()
+{
+  NS_ABORT_IF_FALSE(mRegisteredForXPCOMShutdown,
+                    "unregistering for XPCOM shutdown w/out being registered");
+
+  nsresult rv;
+  nsCOMPtr<nsIObserverService> obsSvc = do_GetService(OBSERVER_SVC_CID, &rv);
+  if (NS_FAILED(rv) ||
+      NS_FAILED(obsSvc->RemoveObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID))) {
+    NS_WARNING("Failed to unregister as observer of XPCOM shutdown");
+  } else {
+    mRegisteredForXPCOMShutdown = PR_FALSE;
   }
 }
 
 void
 SVGDocumentWrapper::FlushLayout()
 {
   nsCOMPtr<nsIPresShell> presShell;
   mViewer->GetPresShell(getter_AddRefs(presShell));

--- a/modules/libpr0n/src/SVGDocumentWrapper.h
+++ b/modules/libpr0n/src/SVGDocumentWrapper.h
@@ -40,16 +40,17 @@
 
 #ifndef mozilla_imagelib_SVGDocumentWrapper_h_
 #define mozilla_imagelib_SVGDocumentWrapper_h_
 
 #include "nsCOMPtr.h"
 #include "nsIStreamListener.h"
 #include "nsIObserver.h"
 #include "nsIDocumentViewer.h"
+#include "nsWeakReference.h"
 
 class nsIAtom;
 class nsIPresShell;
 class nsIRequest;
 class nsIDocumentViewer;
 class nsILoadGroup;
 class nsIFrame;
 class nsIntSize;
@@ -58,17 +59,18 @@ class nsSVGSVGElement;
 #define SVG_MIMETYPE     "image/svg+xml"
 #define OBSERVER_SVC_CID "@mozilla.org/observer-service;1"
 
 
 namespace mozilla {
 namespace imagelib {
 
 class SVGDocumentWrapper : public nsIStreamListener,
-                           public nsIObserver
+                           public nsIObserver,
+                           nsSupportsWeakReference
 {
 public:
   SVGDocumentWrapper();
   ~SVGDocumentWrapper();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSISTREAMLISTENER
   NS_DECL_NSIREQUESTOBSERVER
@@ -160,23 +162,25 @@ public:
   void ResetAnimation();
 
 private:
   nsresult SetupViewer(nsIRequest *aRequest,
                        nsIDocumentViewer** aViewer,
                        nsILoadGroup** aLoadGroup);
   void     DestroyViewer();
   void     RegisterForXPCOMShutdown();
+  void     UnregisterForXPCOMShutdown();
 
   void     FlushLayout();
 
   nsCOMPtr<nsIDocumentViewer> mViewer;
   nsCOMPtr<nsILoadGroup>      mLoadGroup;
   nsCOMPtr<nsIStreamListener> mListener;
   PRPackedBool                mIgnoreInvalidation;
+  PRPackedBool                mRegisteredForXPCOMShutdown;
 
   // Lazily-initialized pointer to nsGkAtoms::svg, to make life easier in
   // non-libxul builds, which don't let us reference nsGkAtoms from imagelib.
   static nsIAtom* kSVGAtom;
 };
 
 } // namespace imagelib
 } // namespace mozilla