diff --git a/layout/style/Rule.h b/layout/style/Rule.h
index f00490efe0de..6b5af0954b13 100644
--- a/layout/style/Rule.h
+++ b/layout/style/Rule.h
@@ -27,8 +27,7 @@ class GroupRule;
 
 #define DECL_STYLE_RULE_INHERIT                            \
   DECL_STYLE_RULE_INHERIT_NO_DOMRULE                       \
-  virtual nsIDOMCSSRule* GetDOMRule() override;        \
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override;
+  virtual nsIDOMCSSRule* GetDOMRule() override;
 
 class Rule : public nsISupports {
 protected:
@@ -113,9 +112,6 @@ public:
   // supposed to have a DOM rule representation (and our code wouldn't work).
   virtual nsIDOMCSSRule* GetDOMRule() = 0;
 
-  // Like GetDOMRule(), but won't create one if we don't have one yet
-  virtual nsIDOMCSSRule* GetExistingDOMRule() = 0;
-
   // to implement methods on nsIDOMCSSRule
   nsresult GetParentRule(nsIDOMCSSRule** aParentRule);
   nsresult GetParentStyleSheet(nsIDOMCSSStyleSheet** aSheet);
diff --git a/layout/style/ServoStyleRule.h b/layout/style/ServoStyleRule.h
index 40d2a2444e3a..03d8f4d43fe9 100644
--- a/layout/style/ServoStyleRule.h
+++ b/layout/style/ServoStyleRule.h
@@ -65,7 +65,6 @@ public:
   int32_t GetType() const final { return css::Rule::STYLE_RULE; }
   already_AddRefed<Rule> Clone() const final;
   nsIDOMCSSRule* GetDOMRule() final { return this; }
-  nsIDOMCSSRule* GetExistingDOMRule() final { return this; }
   size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const final;
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const final;
diff --git a/layout/style/StyleRule.cpp b/layout/style/StyleRule.cpp
index 9e282a3ebbab..d3740f720ba0 100644
--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -1464,12 +1464,6 @@ StyleRule::GetDOMRule()
   return mDOMRule;
 }
 
-/* virtual */ nsIDOMCSSRule*
-StyleRule::GetExistingDOMRule()
-{
-  return mDOMRule;
-}
-
 void
 StyleRule::SetDeclaration(Declaration* aDecl)
 {
diff --git a/layout/style/StyleRule.h b/layout/style/StyleRule.h
index e217c473d088..392164957ac7 100644
--- a/layout/style/StyleRule.h
+++ b/layout/style/StyleRule.h
@@ -348,8 +348,6 @@ public:
 
   virtual nsIDOMCSSRule* GetDOMRule() override;
 
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override;
-
 #ifdef DEBUG
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
@@ -359,6 +357,10 @@ public:
 private:
   ~StyleRule();
 
+  // Drop our references to mDeclaration and mRule, and let them know we're
+  // doing that.
+  void DropReferences();
+
 private:
   nsCSSSelectorList*      mSelector; // null for style attribute
   RefPtr<Declaration>     mDeclaration;
diff --git a/layout/style/nsCSSRules.cpp b/layout/style/nsCSSRules.cpp
index af21220b5762..1530648e7104 100644
--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -43,8 +43,6 @@ using namespace mozilla::dom;
 
 #define IMPL_STYLE_RULE_INHERIT_GET_DOM_RULE_WEAK(class_, super_) \
   /* virtual */ nsIDOMCSSRule* class_::GetDOMRule()               \
-  { return this; }                                                \
-  /* virtual */ nsIDOMCSSRule* class_::GetExistingDOMRule()       \
   { return this; }
 
 #define IMPL_STYLE_RULE_INHERIT(class_, super_) \
@@ -839,7 +837,6 @@ NS_INTERFACE_MAP_BEGIN(DocumentRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSGroupingRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSConditionRule)
   NS_INTERFACE_MAP_ENTRY(nsIDOMCSSMozDocumentRule)
-  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, mozilla::css::Rule)
   NS_DOM_INTERFACE_MAP_ENTRY_CLASSINFO(CSSMozDocumentRule)
 NS_INTERFACE_MAP_END_INHERITING(GroupRule)
 
diff --git a/layout/style/nsCSSRules.h b/layout/style/nsCSSRules.h
index c70f195f65af..9dc792d8f300 100644
--- a/layout/style/nsCSSRules.h
+++ b/layout/style/nsCSSRules.h
@@ -72,10 +72,6 @@ public:
   {
     return this;
   }
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override
-  {
-    return this;
-  }
 
   // nsIDOMCSSRule interface
   NS_DECL_NSIDOMCSSRULE
@@ -127,10 +123,6 @@ public:
   {
     return this;
   }
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override
-  {
-    return this;
-  }
 
   // nsIDOMCSSRule interface
   NS_DECL_NSIDOMCSSRULE
@@ -457,10 +449,6 @@ public:
   {
     return this;
   }
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override
-  {
-    return this;
-  }
 
   // nsIDOMCSSRule interface
   NS_DECL_NSIDOMCSSRULE
@@ -575,10 +563,6 @@ public:
   {
     return this;
   }
-  virtual nsIDOMCSSRule* GetExistingDOMRule() override
-  {
-    return this;
-  }
 
   NS_DECL_ISUPPORTS_INHERITED