--- a/dom/base/nsContentSink.cpp
+++ b/dom/base/nsContentSink.cpp
@@ -102,16 +102,17 @@ nsContentSink::nsContentSink()
   , mLayoutStarted(0)
   , mDynamicLowerValue(0)
   , mParsing(0)
   , mDroppedTimer(0)
   , mDeferredLayoutStart(0)
   , mDeferredFlushTags(0)
   , mIsDocumentObserver(0)
   , mRunsToCompletion(0)
+  , mIsBlockingOnload(false)
   , mDeflectedCount(0)
   , mHasPendingEvent(false)
   , mCurrentParseEndTime(0)
   , mBeginLoadTime(0)
   , mLastSampledUserEventTime(0)
   , mInMonolithicContainer(0)
   , mInNotification(0)
   , mUpdatesInNotification(0)
@@ -1561,18 +1562,24 @@ nsContentSink::DropParserAndPerfHint(voi
   RefPtr<nsParserBase> kungFuDeathGrip(mParser.forget());
 
   if (mDynamicLowerValue) {
     // Reset the performance hint which was set to FALSE
     // when mDynamicLowerValue was set.
     FavorPerformanceHint(true, 0);
   }
 
-  if (!mRunsToCompletion) {
+  // Call UnblockOnload only if mRunsToComletion is false and if
+  // we have already started loading because it's possible that this function
+  // is called (i.e. the parser is terminated) before we start loading due to
+  // destroying the window inside unload event callbacks for the previous
+  // document.
+  if (!mRunsToCompletion && mIsBlockingOnload) {
     mDocument->UnblockOnload(true);
+    mIsBlockingOnload = false;
   }
 }
 
 bool
 nsContentSink::IsScriptExecutingImpl()
 {
   return !!mScriptLoader->GetCurrentScript();
 }
@@ -1617,16 +1624,17 @@ nsContentSink::WillParseImpl(void)
   return NS_OK;
 }
 
 void
 nsContentSink::WillBuildModelImpl()
 {
   if (!mRunsToCompletion) {
     mDocument->BlockOnload();
+    mIsBlockingOnload = true;
 
     mBeginLoadTime = PR_IntervalToMicroseconds(PR_IntervalNow());
   }
 
   mDocument->ResetScrolledToRefAlready();
 
   if (mProcessLinkHeaderEvent.get()) {
     mProcessLinkHeaderEvent.Revoke();

--- a/dom/base/nsContentSink.h
+++ b/dom/base/nsContentSink.h
@@ -305,16 +305,18 @@ protected:
   uint8_t mDeferredFlushTags : 1;
   // If false, we're not ourselves a document observer; that means we
   // shouldn't be performing any more content model notifications,
   // since we're not longer updating our child counts.
   uint8_t mIsDocumentObserver : 1;
   // True if this is parser is a fragment parser or an HTML DOMParser.
   // XML DOMParser leaves this to false for now!
   uint8_t mRunsToCompletion : 1;
+  // True if we are blocking load event.
+  bool mIsBlockingOnload : 1;
 
   //
   // -- Can interrupt parsing members --
   //
 
   // The number of tokens that have been processed since we measured
   // if it's time to return to the main event loop.
   uint32_t mDeflectedCount;