--- a/image/src/Image.cpp
+++ b/image/src/Image.cpp
@@ -5,19 +5,25 @@
 
 #include "nsMimeTypes.h"
 
 #include "Image.h"
 
 namespace mozilla {
 namespace image {
 
+#ifdef DEBUG_imagelib
+static const bool allowOMTURIAccess = true;
+#else
+static const bool allowOMTURIAccess = false;
+#endif
+
 // Constructor
 ImageResource::ImageResource(imgStatusTracker* aStatusTracker, nsIURI* aURI) :
-  mURI(aURI),
+  mURI(new nsMainThreadPtrHolder<nsIURI>(aURI, allowOMTURIAccess)),
   mInnerWindowId(0),
   mAnimationConsumers(0),
   mAnimationMode(kNormalAnimMode),
   mInitialized(false),
   mAnimating(false),
   mError(false)
 {
   if (aStatusTracker) {

--- a/image/src/Image.h
+++ b/image/src/Image.h
@@ -5,16 +5,17 @@
 
 #ifndef MOZILLA_IMAGELIB_IMAGE_H_
 #define MOZILLA_IMAGELIB_IMAGE_H_
 
 #include "mozilla/MemoryReporting.h"
 #include "imgIContainer.h"
 #include "imgStatusTracker.h"
 #include "nsIURI.h"
+#include "nsProxyRelease.h" // for nsMainThreadPtrHolder and nsMainThreadPtrHandle
 
 class nsIRequest;
 class nsIInputStream;
 
 namespace mozilla {
 namespace image {
 
 class Image : public imgIContainer
@@ -151,18 +152,19 @@ public:
   }
   virtual uint64_t InnerWindowID() const MOZ_OVERRIDE { return mInnerWindowId; }
 
   virtual bool HasError() MOZ_OVERRIDE    { return mError; }
   virtual void SetHasError() MOZ_OVERRIDE { mError = true; }
 
   /*
    * Returns a non-AddRefed pointer to the URI associated with this image.
+   * Illegal to use off-main-thread.
    */
-  virtual nsIURI* GetURI() MOZ_OVERRIDE { return mURI; }
+  virtual nsIURI* GetURI() MOZ_OVERRIDE { return mURI.get(); }
 
 protected:
   ImageResource(imgStatusTracker* aStatusTracker, nsIURI* aURI);
 
   // Shared functionality for implementors of imgIContainer. Every
   // implementation of attribute animationMode should forward here.
   nsresult GetAnimationModeInternal(uint16_t *aAnimationMode);
   nsresult SetAnimationModeInternal(uint16_t aAnimationMode);
@@ -180,22 +182,22 @@ protected:
   virtual bool ShouldAnimate() {
     return mAnimationConsumers > 0 && mAnimationMode != kDontAnimMode;
   }
 
   virtual nsresult StartAnimation() = 0;
   virtual nsresult StopAnimation() = 0;
 
   // Member data shared by all implementations of this abstract class
-  nsRefPtr<imgStatusTracker>  mStatusTracker;
-  nsCOMPtr<nsIURI>            mURI;
-  uint64_t                    mInnerWindowId;
-  uint32_t                    mAnimationConsumers;
-  uint16_t                    mAnimationMode;   // Enum values in imgIContainer
-  bool                        mInitialized:1;   // Have we been initalized?
-  bool                        mAnimating:1;     // Are we currently animating?
-  bool                        mError:1;         // Error handling
+  nsRefPtr<imgStatusTracker>    mStatusTracker;
+  nsMainThreadPtrHandle<nsIURI> mURI;
+  uint64_t                      mInnerWindowId;
+  uint32_t                      mAnimationConsumers;
+  uint16_t                      mAnimationMode; // Enum values in imgIContainer
+  bool                          mInitialized:1; // Have we been initalized?
+  bool                          mAnimating:1;   // Are we currently animating?
+  bool                          mError:1;       // Error handling
 };
 
 } // namespace image
 } // namespace mozilla
 
 #endif // MOZILLA_IMAGELIB_IMAGE_H_