diff --git a/mozilla/content/base/public/nsContentUtils.h b/mozilla/content/base/public/nsContentUtils.h
index 35f051118b7..27cc99550bf 100644
--- a/mozilla/content/base/public/nsContentUtils.h
+++ b/mozilla/content/base/public/nsContentUtils.h
@@ -267,6 +267,7 @@ public:
   }
   
   static nsresult GenerateStateKey(nsIContent* aContent,
+                                   nsIDocument* aDocument,
                                    nsIStatefulFrame::SpecialStateID aID,
                                    nsACString& aKey);
 
diff --git a/mozilla/content/base/public/nsIDocument.h b/mozilla/content/base/public/nsIDocument.h
index 2df3255ce89..c3de2d07ade 100644
--- a/mozilla/content/base/public/nsIDocument.h
+++ b/mozilla/content/base/public/nsIDocument.h
@@ -88,10 +88,10 @@ class nsHTMLStyleSheet;
 class nsIHTMLCSSStyleSheet;
 
 // IID for the nsIDocument interface
-// c59c70e5-28d1-494b-9f8e-c18368d09ebc
+// c76fbc2d-2dca-4ce1-b6e9-7f7030b01e17
 #define NS_IDOCUMENT_IID      \
-{ 0x9f670164, 0xc446, 0x11d8, \
-  { 0x84, 0xe1, 0x00, 0x0a, 0x95, 0xdc, 0x23, 0x4c } }
+{ 0xc76fbc2d, 0x2dca, 0x4ce1, \
+  { 0xb6, 0xe9, 0x7f, 0x70, 0x30, 0xb0, 0x1e, 0x17 } }
 
 // The base value for the content ID counter.
 // This counter is used by the document to 
@@ -116,7 +116,8 @@ public:
   nsIDocument()
     : mCharacterSet(NS_LITERAL_CSTRING("ISO-8859-1")),
       mNextContentID(NS_CONTENT_ID_COUNTER_BASE),
-      mNodeInfoManager(nsnull)
+      mNodeInfoManager(nsnull),
+      mPartID(0)
   {
   }
 
@@ -635,6 +636,20 @@ public:
 
   nsPropertyTable* PropertyTable() { return &mPropertyTable; }
 
+  /**
+   * Sets the ID used to identify this part of the multipart document
+   */
+  void SetPartID(PRUint32 aID) {
+    mPartID = aID;
+  }
+
+  /**
+   * Return the ID used to identify this part of the multipart document
+   */
+  PRUint32 GetPartID() const {
+    return mPartID;
+  }
+
 protected:
   ~nsIDocument()
   {
@@ -680,6 +695,10 @@ protected:
 
   // The document's security info
   nsCOMPtr<nsISupports> mSecurityInfo;
+
+  // if this document is part of a multipart document,
+  // the ID can be used to distinguish it from the other parts.
+  PRUint32 mPartID;
 };
 
 
diff --git a/mozilla/content/base/src/nsContentUtils.cpp b/mozilla/content/base/src/nsContentUtils.cpp
index d8079f4f647..406a218a36f 100644
--- a/mozilla/content/base/src/nsContentUtils.cpp
+++ b/mozilla/content/base/src/nsContentUtils.cpp
@@ -1387,14 +1387,18 @@ static inline PRBool IsAutocompleteOff(nsIDOMElement* aElement)
 
 /*static*/ nsresult
 nsContentUtils::GenerateStateKey(nsIContent* aContent,
+                                 nsIDocument* aDocument,
                                  nsIStatefulFrame::SpecialStateID aID,
                                  nsACString& aKey)
 {
   aKey.Truncate();
 
+  PRUint32 partID = aDocument ? aDocument->GetPartID() : 0;
+
   // SpecialStateID case - e.g. scrollbars around the content window
   // The key in this case is the special state id (always < min(contentID))
   if (nsIStatefulFrame::eNoID != aID) {
+    KeyAppendInt(partID, aKey);  // first append a partID
     KeyAppendInt(aID, aKey);
     return NS_OK;
   }
@@ -1415,6 +1419,7 @@ nsContentUtils::GenerateStateKey(nsIContent* aContent,
 
   nsCOMPtr<nsIHTMLDocument> htmlDocument(do_QueryInterface(aContent->GetDocument()));
 
+  KeyAppendInt(partID, aKey);  // first append a partID
   PRBool generatedUniqueKey = PR_FALSE;
 
   if (htmlDocument) {
diff --git a/mozilla/content/html/content/src/nsGenericHTMLElement.cpp b/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
index 9f80ecddece..5bb19617f6b 100644
--- a/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
@@ -2320,7 +2320,7 @@ nsGenericHTMLElement::GetLayoutHistoryAndKey(nsGenericHTMLElement* aContent,
   //
   // Get the state key
   //
-  rv = nsContentUtils::GenerateStateKey(aContent, nsIStatefulFrame::eNoID, aKey);
+  rv = nsContentUtils::GenerateStateKey(aContent, doc, nsIStatefulFrame::eNoID, aKey);
   NS_ENSURE_SUCCESS(rv, rv);
 
   // If the state key is blank, this is anonymous content or for
diff --git a/mozilla/docshell/base/nsDocShell.cpp b/mozilla/docshell/base/nsDocShell.cpp
index d43ba133c54..cf3f9d6be1c 100644
--- a/mozilla/docshell/base/nsDocShell.cpp
+++ b/mozilla/docshell/base/nsDocShell.cpp
@@ -4705,6 +4705,22 @@ nsDocShell::CreateContentViewer(const char *aContentType,
 
     mEODForCurrentDocument = PR_FALSE;
 
+    // if this document is part of a multipart document,
+    // the ID can be used to distinguish it from the other parts.
+    nsCOMPtr<nsIMultiPartChannel> multiPartChannel(do_QueryInterface(request));
+    if (multiPartChannel) {
+      nsCOMPtr<nsIPresShell> shell;
+      rv = GetPresShell(getter_AddRefs(shell));
+      if (NS_SUCCEEDED(rv) && shell) {
+        nsIDocument *doc = shell->GetDocument();
+        if (doc) {
+          PRUint32 partID;
+          multiPartChannel->GetPartID(&partID);
+          doc->SetPartID(partID);
+        }
+      }
+    }
+
     // Give hint to native plevent dispatch mechanism. If a document
     // is loading the native plevent dispatch mechanism should favor
     // performance over normal native event dispatch priorities.
diff --git a/mozilla/layout/base/nsFrameManager.cpp b/mozilla/layout/base/nsFrameManager.cpp
index e1ecac8c029..efe28eae489 100644
--- a/mozilla/layout/base/nsFrameManager.cpp
+++ b/mozilla/layout/base/nsFrameManager.cpp
@@ -1719,7 +1719,9 @@ nsFrameManager::CaptureFrameStateFor(nsIFrame* aFrame,
   // Generate the hash key to store the state under
   // Exit early if we get empty key
   nsCAutoString stateKey;
-  rv = nsContentUtils::GenerateStateKey(aFrame->GetContent(), aID, stateKey);
+  nsIContent* content = aFrame->GetContent();
+  nsIDocument* doc = content ? content->GetCurrentDoc() : nsnull;
+  rv = nsContentUtils::GenerateStateKey(content, doc, aID, stateKey);
   if(NS_FAILED(rv) || stateKey.IsEmpty()) {
     return;
   }
@@ -1779,7 +1781,8 @@ nsFrameManager::RestoreFrameStateFor(nsIFrame* aFrame,
   }
 
   nsCAutoString stateKey;
-  nsresult rv = nsContentUtils::GenerateStateKey(content, aID, stateKey);
+  nsIDocument* doc = content->GetCurrentDoc();
+  nsresult rv = nsContentUtils::GenerateStateKey(content, doc, aID, stateKey);
   if (NS_FAILED(rv) || stateKey.IsEmpty()) {
     return;
   }
diff --git a/mozilla/netwerk/base/public/nsIMultiPartChannel.idl b/mozilla/netwerk/base/public/nsIMultiPartChannel.idl
index f6aac4ee26b..d8d58833a31 100644
--- a/mozilla/netwerk/base/public/nsIMultiPartChannel.idl
+++ b/mozilla/netwerk/base/public/nsIMultiPartChannel.idl
@@ -45,7 +45,7 @@ interface nsIChannel;
  * associated with a MultiPartChannel.
  */
 
-[scriptable, uuid(62d77f66-8ad0-4a7f-91a1-bb048b136490)]
+[scriptable, uuid(c1284456-652d-4130-a1a4-48a652af2330)]
 interface nsIMultiPartChannel : nsISupports
 {
     /**
@@ -59,4 +59,10 @@ interface nsIMultiPartChannel : nsISupports
      * handling method, preferred filename, etc.  See RFC 2183.
      */
     attribute ACString contentDisposition;
+
+    /**
+     * Attribute guaranteed to be different for different parts of
+     * the same multipart document.
+     */
+    readonly attribute PRUint32 partID;
 };
diff --git a/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.cpp b/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.cpp
index 87d76ecc862..0ddfe65aff6 100644
--- a/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.cpp
+++ b/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.cpp
@@ -79,7 +79,7 @@ class nsPartChannel : public nsIChannel,
                       public nsIMultiPartChannel
 {
 public:
-  nsPartChannel(nsIChannel *aMultipartChannel);
+  nsPartChannel(nsIChannel *aMultipartChannel, PRUint32 aPartID);
 
   void InitializeByteRange(PRInt64 aStart, PRInt64 aEnd);
 
@@ -108,14 +108,18 @@ protected:
   PRBool                  mIsByteRangeRequest;
   nsInt64                 mByteRangeStart;
   nsInt64                 mByteRangeEnd;
+
+  PRUint32                mPartID; // unique ID that can be used to identify
+                                   // this part of the multipart document
 };
 
-nsPartChannel::nsPartChannel(nsIChannel *aMultipartChannel) :
+nsPartChannel::nsPartChannel(nsIChannel *aMultipartChannel, PRUint32 aPartID) :
   mStatus(NS_OK),
   mContentLength(LL_MAXUINT),
   mIsByteRangeRequest(PR_FALSE),
   mByteRangeStart(0),
-  mByteRangeEnd(0)
+  mByteRangeEnd(0),
+  mPartID(aPartID)
 {
     mMultipartChannel = aMultipartChannel;
 
@@ -367,6 +371,13 @@ nsPartChannel::SetContentDisposition(const nsACString &aContentDisposition)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsPartChannel::GetPartID(PRUint32 *aPartID)
+{
+    *aPartID = mPartID;
+    return NS_OK;
+}
+
 //
 // nsIByteRangeRequest implementation...
 //
@@ -717,7 +728,9 @@ nsMultiMixedConv::OnStopRequest(nsIRequest *request, nsISupports *ctxt,
 
 
 // nsMultiMixedConv methods
-nsMultiMixedConv::nsMultiMixedConv() {
+nsMultiMixedConv::nsMultiMixedConv() :
+  mCurrentPartID(0)
+{
     mTokenLen           = 0;
     mNewPart            = PR_TRUE;
     mContentLength      = LL_MAXUINT;
@@ -764,7 +777,7 @@ nsMultiMixedConv::SendStart(nsIChannel *aChannel) {
     NS_ASSERTION(!mPartChannel, "tisk tisk, shouldn't be overwriting a channel");
 
     nsPartChannel *newChannel;
-    newChannel = new nsPartChannel(aChannel);
+    newChannel = new nsPartChannel(aChannel, mCurrentPartID++);
     if (!newChannel)
         return NS_ERROR_OUT_OF_MEMORY;
 
diff --git a/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.h b/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.h
index a9525a30b5f..ab698d5b81c 100644
--- a/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.h
+++ b/mozilla/netwerk/streamconv/converters/nsMultiMixedConv.h
@@ -137,6 +137,8 @@ protected:
     nsInt64             mByteRangeStart;
     nsInt64             mByteRangeEnd;
     PRBool              mIsByteRangeRequest;
+
+    PRUint32            mCurrentPartID;
 };
 
 #endif /* __nsmultimixedconv__h__ */