diff --git a/dom/base/Selection.cpp b/dom/base/Selection.cpp
index 11e0870125db..ce0bc78a8c2b 100644
--- a/dom/base/Selection.cpp
+++ b/dom/base/Selection.cpp
@@ -2104,6 +2104,10 @@ Selection::RemoveAllRangesTemporarily()
   RemoveAllRanges(result);
   if (result.Failed()) {
     mCachedRange = nullptr;
+  } else if (mCachedRange) {
+    // To save the computing cost to keep valid DOM point against DOM tree
+    // changes, we should clear the range temporarily.
+    mCachedRange->ResetTemporarily();
   }
   return result.StealNSResult();
 }
diff --git a/dom/base/Selection.h b/dom/base/Selection.h
index 13c043800fdc..5d2c26689df8 100644
--- a/dom/base/Selection.h
+++ b/dom/base/Selection.h
@@ -662,6 +662,12 @@ private:
   // released by Clear(), RemoveAllRangesTemporarily() stores it with this.
   // If Collapse() is called without existing ranges, it'll reuse this range
   // for saving the creation cost.
+  // Note that while the range is cached by this, we keep the range being
+  // a mutation observer because it is not so cheap to register the range
+  // as a mutation observer again.  On the other hand, we make it not
+  // positioned because it is not so cheap to keep valid DOM point against
+  // mutations.  This does not cause any problems because we will set new
+  // DOM point when we treat it as a range of Selection again.
   RefPtr<nsRange> mCachedRange;
   RefPtr<nsFrameSelection> mFrameSelection;
   RefPtr<nsAutoScrollTimer> mAutoScrollTimer;
diff --git a/dom/base/nsRange.cpp b/dom/base/nsRange.cpp
index c3aa91249808..2bd6890734ba 100644
--- a/dom/base/nsRange.cpp
+++ b/dom/base/nsRange.cpp
@@ -491,9 +491,15 @@ void
 nsRange::CharacterDataChanged(nsIContent* aContent,
                               const CharacterDataChangeInfo& aInfo)
 {
+  // If this is called when this is not positioned, it means that this range
+  // will be initialized again or destroyed soon.  See Selection::mCachedRange.
+  if (!mIsPositioned) {
+    MOZ_ASSERT(mRoot);
+    return;
+  }
+
   MOZ_ASSERT(!mNextEndRef);
   MOZ_ASSERT(!mNextStartRef);
-  MOZ_ASSERT(mIsPositioned, "shouldn't be notified if not positioned");
 
   nsINode* newRoot = nullptr;
   RawRangeBoundary newStart;
@@ -645,7 +651,12 @@ nsRange::CharacterDataChanged(nsIContent* aContent,
 void
 nsRange::ContentAppended(nsIContent*  aFirstNewContent)
 {
-  NS_ASSERTION(mIsPositioned, "shouldn't be notified if not positioned");
+  // If this is called when this is not positioned, it means that this range
+  // will be initialized again or destroyed soon.  See Selection::mCachedRange.
+  if (!mIsPositioned) {
+    MOZ_ASSERT(mRoot);
+    return;
+  }
 
   nsINode* container = aFirstNewContent->GetParentNode();
   MOZ_ASSERT(container);
@@ -680,7 +691,12 @@ nsRange::ContentAppended(nsIContent*  aFirstNewContent)
 void
 nsRange::ContentInserted(nsIContent* aChild)
 {
-  MOZ_ASSERT(mIsPositioned, "shouldn't be notified if not positioned");
+  // If this is called when this is not positioned, it means that this range
+  // will be initialized again or destroyed soon.  See Selection::mCachedRange.
+  if (!mIsPositioned) {
+    MOZ_ASSERT(mRoot);
+    return;
+  }
 
   bool updateBoundaries = false;
   nsINode* container = aChild->GetParentNode();
@@ -730,7 +746,13 @@ nsRange::ContentInserted(nsIContent* aChild)
 void
 nsRange::ContentRemoved(nsIContent* aChild, nsIContent* aPreviousSibling)
 {
-  MOZ_ASSERT(mIsPositioned, "shouldn't be notified if not positioned");
+  // If this is called when this is not positioned, it means that this range
+  // will be initialized again or destroyed soon.  See Selection::mCachedRange.
+  if (!mIsPositioned) {
+    MOZ_ASSERT(mRoot);
+    return;
+  }
+
   nsINode* container = aChild->GetParentNode();
   MOZ_ASSERT(container);
 
@@ -937,17 +959,17 @@ nsRange::DoSetRange(const RawRangeBoundary& aStart,
                     nsINode* aRoot, bool aNotInsertedYet)
 {
   MOZ_ASSERT((aStart.IsSet() && aEnd.IsSet() && aRoot) ||
-             (!aStart.IsSet() && !aEnd.IsSet() && !aRoot),
+             (!aStart.IsSet() && !aEnd.IsSet()),
              "Set all or none");
 
-  MOZ_ASSERT(!aRoot || aNotInsertedYet ||
+  MOZ_ASSERT(!aRoot || (!aStart.IsSet() && !aEnd.IsSet()) || aNotInsertedYet ||
              (nsContentUtils::ContentIsDescendantOf(aStart.Container(), aRoot) &&
               nsContentUtils::ContentIsDescendantOf(aEnd.Container(), aRoot) &&
               aRoot == IsValidBoundary(aStart.Container()) &&
               aRoot == IsValidBoundary(aEnd.Container())),
              "Wrong root");
 
-  MOZ_ASSERT(!aRoot ||
+  MOZ_ASSERT(!aRoot || (!aStart.IsSet() && !aEnd.IsSet()) ||
              (aStart.Container()->IsContent() &&
               aEnd.Container()->IsContent() &&
               aRoot ==
diff --git a/dom/base/nsRange.h b/dom/base/nsRange.h
index e52bb4147096..639b0108439a 100644
--- a/dom/base/nsRange.h
+++ b/dom/base/nsRange.h
@@ -158,6 +158,17 @@ public:
   nsINode* GetCommonAncestor() const;
   void Reset();
 
+  /**
+   * ResetTemporarily() is called when Selection starts to cache the instance
+   * to reuse later.  This method clears mStart, mEnd and mIsPositioned but
+   * does not clear mRoot for reducing the cost to register this as a mutation
+   * observer again.
+   */
+  void ResetTemporarily()
+  {
+    DoSetRange(RawRangeBoundary(), RawRangeBoundary(), mRoot);
+  }
+
   /**
    * SetStart() and SetEnd() sets start point or end point separately.
    * However, this is expensive especially when it's a range of Selection.