diff --git a/parser/html/nsHtml5Highlighter.cpp b/parser/html/nsHtml5Highlighter.cpp
index 892918920f3..c7c509ab94a 100644
--- a/parser/html/nsHtml5Highlighter.cpp
+++ b/parser/html/nsHtml5Highlighter.cpp
@@ -45,6 +45,10 @@
 
 using namespace mozilla;
 
+// The old code had a limit of 16 tokens. 1300 is a number picked my measuring
+// the size of 16 tokens on cnn.com.
+#define NS_HTML5_HIGHLIGHTER_PRE_BREAK_THRESHOLD 1300
+
 PRUnichar nsHtml5Highlighter::sComment[] =
   { 'c', 'o', 'm', 'm', 'e', 'n', 't', 0 };
 
@@ -78,6 +82,7 @@ nsHtml5Highlighter::nsHtml5Highlighter(nsAHtml5TreeOpSink* aOpSink)
  , mCStart(PR_INT32_MAX)
  , mPos(0)
  , mLineNumber(1)
+ , mUnicharsInThisPre(0)
  , mInlinesOpen(0)
  , mInCharacters(false)
  , mBuffer(nsnull)
@@ -619,10 +624,25 @@ nsHtml5Highlighter::FlushChars()
         case '\n': {
           ++i;
           if (mCStart < i) {
-            AppendCharacters(buf, mCStart, i - mCStart);
+            PRInt32 len = i - mCStart;
+            AppendCharacters(buf, mCStart, len);
             mCStart = i;
+            mUnicharsInThisPre += len;
           }
           ++mLineNumber;
+          if (mUnicharsInThisPre > NS_HTML5_HIGHLIGHTER_PRE_BREAK_THRESHOLD &&
+              !mInlinesOpen && mInCharacters) {
+            mUnicharsInThisPre = 0;
+            // Split the pre. See bug 86355.
+            Pop(); // span
+            Pop(); // pre
+            Push(nsGkAtoms::pre, nsnull);
+            nsHtml5TreeOperation* treeOp = mOpQueue.AppendElement();
+            NS_ASSERTION(treeOp, "Tree op allocation failed.");
+            treeOp->InitAddLineNumberId(CurrentNode(), mLineNumber);
+            Push(nsGkAtoms::span, nsnull);
+            break;
+          }
           Push(nsGkAtoms::span, nsnull);
           nsHtml5TreeOperation* treeOp = mOpQueue.AppendElement();
           NS_ASSERTION(treeOp, "Tree op allocation failed.");
@@ -636,8 +656,10 @@ nsHtml5Highlighter::FlushChars()
       }
     }
     if (mCStart < mPos) {
-      AppendCharacters(buf, mCStart, mPos - mCStart);
+      PRInt32 len = mPos - mCStart;
+      AppendCharacters(buf, mCStart, len);
       mCStart = mPos;
+      mUnicharsInThisPre += len;
     }
   }
 }
diff --git a/parser/html/nsHtml5Highlighter.h b/parser/html/nsHtml5Highlighter.h
index 8007664528d..3def948c9d5 100644
--- a/parser/html/nsHtml5Highlighter.h
+++ b/parser/html/nsHtml5Highlighter.h
@@ -316,6 +316,12 @@ class nsHtml5Highlighter
      */
     PRInt32 mLineNumber;
 
+    /**
+     * The number of PRUnichars flushed since the start of the current pre
+     * block.
+     */
+    PRInt32 mUnicharsInThisPre;
+
     /**
      * The number of inline elements open inside the <pre> excluding the
      * span potentially wrapping a run of characters.