diff --git a/dom/xbl/nsXBLPrototypeResources.cpp b/dom/xbl/nsXBLPrototypeResources.cpp
index 0cd725e60dc6..b43ebaba4124 100644
--- a/dom/xbl/nsXBLPrototypeResources.cpp
+++ b/dom/xbl/nsXBLPrototypeResources.cpp
@@ -78,7 +78,6 @@ nsXBLPrototypeResources::FlushSkinSheets()
   // We have scoped stylesheets.  Reload any chrome stylesheets we
   // encounter.  (If they aren't skin sheets, it doesn't matter, since
   // they'll still be in the chrome cache.
-  mRuleProcessor = nullptr;
 
   nsTArray<nsRefPtr<CSSStyleSheet>> oldSheets;
 
@@ -144,7 +143,8 @@ nsXBLPrototypeResources::GatherRuleProcessor()
 {
   mRuleProcessor = new nsCSSRuleProcessor(mStyleSheetList,
                                           nsStyleSet::eDocSheet,
-                                          nullptr);
+                                          nullptr,
+                                          mRuleProcessor);
 }
 
 void
diff --git a/layout/style/nsCSSRuleProcessor.cpp b/layout/style/nsCSSRuleProcessor.cpp
index f17026f58393..1f363565f749 100644
--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -1087,7 +1087,9 @@ RuleCascadeData::AttributeListFor(nsIAtom* aAttribute)
 
 nsCSSRuleProcessor::nsCSSRuleProcessor(const sheet_array_type& aSheets,
                                        uint8_t aSheetType,
-                                       Element* aScopeElement)
+                                       Element* aScopeElement,
+                                       nsCSSRuleProcessor*
+                                         aPreviousCSSRuleProcessor)
   : mSheets(aSheets)
   , mRuleCascades(nullptr)
   , mLastPresContext(nullptr)
diff --git a/layout/style/nsCSSRuleProcessor.h b/layout/style/nsCSSRuleProcessor.h
index d711fe83579a..46b44fe1be21 100644
--- a/layout/style/nsCSSRuleProcessor.h
+++ b/layout/style/nsCSSRuleProcessor.h
@@ -52,9 +52,12 @@ public:
 
   // aScopeElement must be non-null iff aSheetType is
   // nsStyleSet::eScopedDocSheet.
+  // aPreviousCSSRuleProcessor is the rule processor (if any) that this
+  // one is replacing.
   nsCSSRuleProcessor(const sheet_array_type& aSheets,
                      uint8_t aSheetType,
-                     mozilla::dom::Element* aScopeElement);
+                     mozilla::dom::Element* aScopeElement,
+                     nsCSSRuleProcessor* aPreviousCSSRuleProcessor);
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_CYCLE_COLLECTION_CLASS(nsCSSRuleProcessor)
diff --git a/layout/style/nsStyleSet.cpp b/layout/style/nsStyleSet.cpp
index 20029c924323..bb9f219f5776 100644
--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -352,6 +352,9 @@ SortStyleSheetsByScope(nsTArray<CSSStyleSheet*>& aSheets)
 nsresult
 nsStyleSet::GatherRuleProcessors(sheetType aType)
 {
+  nsCOMPtr<nsIStyleRuleProcessor> oldRuleProcessor(mRuleProcessors[aType]);
+  nsTArray<nsCOMPtr<nsIStyleRuleProcessor>> oldScopedDocRuleProcessors;
+
   mRuleProcessors[aType] = nullptr;
   if (aType == eScopedDocSheet) {
     for (uint32_t i = 0; i < mScopedDocSheetRuleProcessors.Length(); i++) {
@@ -360,7 +363,9 @@ nsStyleSet::GatherRuleProcessors(sheetType aType)
         static_cast<nsCSSRuleProcessor*>(processor)->GetScopeElement();
       scope->ClearIsScopedStyleRoot();
     }
-    mScopedDocSheetRuleProcessors.Clear();
+
+    // Clear mScopedDocSheetRuleProcessors, but save it.
+    oldScopedDocRuleProcessors.SwapElements(mScopedDocSheetRuleProcessors);
   }
   if (mAuthorStyleDisabled && (aType == eDocSheet || 
                                aType == eScopedDocSheet ||
@@ -420,6 +425,21 @@ nsStyleSet::GatherRuleProcessors(sheetType aType)
       // adjacent and that ancestor scopes come before descendent scopes.
       SortStyleSheetsByScope(sheets);
 
+      // Put the old scoped rule processors in a hashtable so that we
+      // can retrieve them efficiently, even in edge cases like the
+      // simultaneous removal and addition of a large number of elements
+      // with scoped sheets.
+      nsDataHashtable<nsPtrHashKey<Element>,
+                      nsCSSRuleProcessor*> oldScopedRuleProcessorHash;
+      for (size_t i = oldScopedDocRuleProcessors.Length(); i-- != 0; ) {
+        nsCSSRuleProcessor* oldRP =
+          static_cast<nsCSSRuleProcessor*>(oldScopedDocRuleProcessors[i].get());
+        Element* scope = oldRP->GetScopeElement();
+        MOZ_ASSERT(!oldScopedRuleProcessorHash.Get(scope),
+                   "duplicate rule processors for same scope element?");
+        oldScopedRuleProcessorHash.Put(scope, oldRP);
+      }
+
       uint32_t start = 0, end;
       do {
         // Find the range of style sheets with the same scope.
@@ -434,8 +454,10 @@ nsStyleSet::GatherRuleProcessors(sheetType aType)
         // Create a rule processor for the scope.
         nsTArray<nsRefPtr<CSSStyleSheet>> sheetsForScope;
         sheetsForScope.AppendElements(sheets.Elements() + start, end - start);
+        nsCSSRuleProcessor* oldRP = oldScopedRuleProcessorHash.Get(scope);
         mScopedDocSheetRuleProcessors.AppendElement
-          (new nsCSSRuleProcessor(sheetsForScope, uint8_t(aType), scope));
+          (new nsCSSRuleProcessor(sheetsForScope, uint8_t(aType), scope,
+                                  oldRP));
 
         start = end;
       } while (start < count);
@@ -457,7 +479,9 @@ nsStyleSet::GatherRuleProcessors(sheetType aType)
           cssSheets.AppendElement(cssSheet);
         }
         mRuleProcessors[aType] =
-          new nsCSSRuleProcessor(cssSheets, uint8_t(aType), nullptr);
+          new nsCSSRuleProcessor(cssSheets, uint8_t(aType), nullptr,
+                                 static_cast<nsCSSRuleProcessor*>(
+                                   oldRuleProcessor.get()));
       } break;
 
       default: