--- a/devtools/server/actors/object.js
+++ b/devtools/server/actors/object.js
@@ -1733,17 +1733,17 @@ DebuggerServer.ObjectActorPreviewers.Obj
     grip.preview = {
       kind: "ObjectWithText",
       text: hooks.createValueGrip(rawObj.selectorText),
     };
     return true;
   },
 
   function ObjectWithURL({obj, hooks}, grip, rawObj) {
-    if (isWorker || !rawObj || !(rawObj instanceof Ci.nsIDOMCSSImportRule ||
+    if (isWorker || !rawObj || !(obj.class == "CSSImportRule" ||
                                  rawObj instanceof Ci.nsIDOMCSSStyleSheet ||
                                  obj.class == "Location" ||
                                  rawObj instanceof Ci.nsIDOMWindow)) {
       return false;
     }
 
     let url;
     if (rawObj instanceof Ci.nsIDOMWindow && rawObj.location) {

--- a/devtools/server/actors/styles.js
+++ b/devtools/server/actors/styles.js
@@ -1309,18 +1309,17 @@ var StyleRuleActor = protocol.ActorClass
     // effects that pop up based on property values that were already set on the
     // element.
 
     let document;
     if (this.rawNode) {
       document = this.rawNode.ownerDocument;
     } else {
       let parentStyleSheet = this._parentSheet;
-      while (parentStyleSheet.ownerRule &&
-          parentStyleSheet.ownerRule instanceof Ci.nsIDOMCSSImportRule) {
+      while (parentStyleSheet.ownerRule) {
         parentStyleSheet = parentStyleSheet.ownerRule.parentStyleSheet;
       }
 
       document = this.getDocument(parentStyleSheet);
     }
 
     let tempElement = document.createElementNS(XHTML_NS, "div");
 

--- a/dom/interfaces/css/moz.build
+++ b/dom/interfaces/css/moz.build
@@ -4,17 +4,16 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 with Files("**"):
     BUG_COMPONENT = ("Core", "DOM: CSS Object Model")
 
 XPIDL_SOURCES += [
     'nsIDOMCounter.idl',
-    'nsIDOMCSSImportRule.idl',
     'nsIDOMCSSKeyframeRule.idl',
     'nsIDOMCSSKeyframesRule.idl',
     'nsIDOMCSSPageRule.idl',
     'nsIDOMCSSPrimitiveValue.idl',
     'nsIDOMCSSRule.idl',
     'nsIDOMCSSRuleList.idl',
     'nsIDOMCSSStyleDeclaration.idl',
     'nsIDOMCSSStyleRule.idl',

deleted file mode 100644
--- a/dom/interfaces/css/nsIDOMCSSImportRule.idl
+++ /dev/null
@@ -1,17 +0,0 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "nsISupports.idl"
-
-interface nsIDOMMediaList;
-interface nsIDOMCSSStyleSheet;
-
-[scriptable, uuid(d3b2b914-01ef-4663-beda-a6475a26f491)]
-interface nsIDOMCSSImportRule : nsISupports
-{
-  readonly attribute DOMString           href;
-  readonly attribute nsIDOMMediaList     media;
-  readonly attribute nsIDOMCSSStyleSheet styleSheet;
-};

--- a/layout/style/CSSImportRule.cpp
+++ b/layout/style/CSSImportRule.cpp
@@ -7,47 +7,23 @@
 #include "mozilla/dom/CSSImportRule.h"
 
 #include "mozilla/dom/CSSImportRuleBinding.h"
 #include "mozilla/dom/MediaList.h"
 
 namespace mozilla {
 namespace dom {
 
-NS_IMPL_ADDREF_INHERITED(CSSImportRule, css::Rule)
-NS_IMPL_RELEASE_INHERITED(CSSImportRule, css::Rule)
-
-// QueryInterface implementation for CSSImportRule
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CSSImportRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSImportRule)
-NS_INTERFACE_MAP_END_INHERITING(css::Rule)
-
 bool
 CSSImportRule::IsCCLeaf() const
 {
   // We're not a leaf.
   return false;
 }
 
-NS_IMETHODIMP
-CSSImportRule::GetMedia(nsIDOMMediaList** aMedia)
-{
-  NS_ENSURE_ARG_POINTER(aMedia);
-  NS_IF_ADDREF(*aMedia = GetMedia());
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-CSSImportRule::GetStyleSheet(nsIDOMCSSStyleSheet** aStyleSheet)
-{
-  NS_ENSURE_ARG_POINTER(aStyleSheet);
-  NS_IF_ADDREF(*aStyleSheet = GetStyleSheet());
-  return NS_OK;
-}
-
 /* virtual */ JSObject*
 CSSImportRule::WrapObject(JSContext* aCx,
                           JS::Handle<JSObject*> aGivenProto)
 {
   return CSSImportRuleBinding::Wrap(aCx, this, aGivenProto);
 }
 
 } // namespace dom

--- a/layout/style/CSSImportRule.h
+++ b/layout/style/CSSImportRule.h
@@ -3,45 +3,38 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef mozilla_dom_CSSImportRule_h
 #define mozilla_dom_CSSImportRule_h
 
 #include "mozilla/css/Rule.h"
-#include "nsIDOMCSSImportRule.h"
 
 namespace mozilla {
 namespace dom {
 
 class CSSImportRule : public css::Rule
-                    , public nsIDOMCSSImportRule
 {
 protected:
   using Rule::Rule;
   virtual ~CSSImportRule() {}
 
 public:
-  NS_DECL_ISUPPORTS_INHERITED
   bool IsCCLeaf() const final;
 
   int32_t GetType() const final { return css::Rule::IMPORT_RULE; }
   using Rule::GetType;
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf)
     const override = 0;
 
-  // nsIDOMCSSImportRule interface
-  NS_IMETHOD GetMedia(nsIDOMMediaList** aMedia) final;
-  NS_IMETHOD GetStyleSheet(nsIDOMCSSStyleSheet** aStyleSheet) final;
-
   // WebIDL interface
   uint16_t Type() const final { return nsIDOMCSSRule::IMPORT_RULE; }
-  // The XPCOM GetHref is fine, since it never fails.
+  virtual void GetHref(nsAString& aHref) const = 0;
   virtual dom::MediaList* GetMedia() const = 0;
   virtual StyleSheet* GetStyleSheet() const = 0;
 
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
 };
 
 } // namespace dom

--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -144,28 +144,19 @@ CSSStyleSheet::RebuildChildList(css::Rul
     return true;
   }
 
   if (type != css::Rule::IMPORT_RULE) {
     // We're past all the import rules; stop the enumeration.
     return false;
   }
 
-  // XXXbz We really need to decomtaminate all this stuff.  Is there a reason
-  // that I can't just QI to ImportRule and get a CSSStyleSheet
-  // directly from it?
-  nsCOMPtr<nsIDOMCSSImportRule> importRule(do_QueryInterface(aRule));
-  NS_ASSERTION(importRule, "GetType lied");
+  css::ImportRule* importRule = static_cast<css::ImportRule*>(aRule);
+  StyleSheet* sheet = importRule->GetStyleSheet();
 
-  nsCOMPtr<nsIDOMCSSStyleSheet> childSheet;
-  importRule->GetStyleSheet(getter_AddRefs(childSheet));
-
-  // Have to do this QI to be safe, since XPConnect can fake
-  // nsIDOMCSSStyleSheets
-  RefPtr<CSSStyleSheet> sheet = do_QueryObject(childSheet);
   if (!sheet) {
     return true;
   }
 
   (*aBuilder->sheetSlot) = sheet;
   aBuilder->SetParentLinks(*aBuilder->sheetSlot);
   aBuilder->sheetSlot = &(*aBuilder->sheetSlot)->mNext;
   return true;
@@ -893,25 +884,20 @@ CSSStyleSheet::ReparseSheet(const nsAStr
   // detach existing rules (including child sheets via import rules)
   css::LoaderReusableStyleSheets reusableSheets;
   int ruleCount;
   while ((ruleCount = Inner()->mOrderedRules.Count()) != 0) {
     RefPtr<css::Rule> rule = Inner()->mOrderedRules.ObjectAt(ruleCount - 1);
     Inner()->mOrderedRules.RemoveObjectAt(ruleCount - 1);
     rule->SetStyleSheet(nullptr);
     if (rule->GetType() == css::Rule::IMPORT_RULE) {
-      nsCOMPtr<nsIDOMCSSImportRule> importRule(do_QueryInterface(rule));
-      NS_ASSERTION(importRule, "GetType lied");
-
-      nsCOMPtr<nsIDOMCSSStyleSheet> childSheet;
-      importRule->GetStyleSheet(getter_AddRefs(childSheet));
-
-      RefPtr<CSSStyleSheet> cssSheet = do_QueryObject(childSheet);
-      if (cssSheet && cssSheet->GetOriginalURI()) {
-        reusableSheets.AddReusableSheet(cssSheet);
+      auto importRule = static_cast<css::ImportRule*>(rule.get());
+      RefPtr<StyleSheet> sheet = importRule->GetStyleSheet();
+      if (sheet && sheet->GetOriginalURI()) {
+        reusableSheets.AddReusableSheet(sheet);
       }
     }
     RuleRemoved(*rule);
   }
 
   // nuke child sheets list and current namespace map
   for (StyleSheet* child = GetFirstChild(); child; ) {
     NS_ASSERTION(child->mParent == this, "Child sheet is not parented to this!");

--- a/layout/style/ImportRule.h
+++ b/layout/style/ImportRule.h
@@ -34,38 +34,32 @@ class ImportRule final : public dom::CSS
 public:
   ImportRule(nsMediaList* aMedia, const nsString& aURLSpec,
              uint32_t aLineNumber, uint32_t aColumnNumber);
 private:
   // for |Clone|
   ImportRule(const ImportRule& aCopy);
   ~ImportRule();
 public:
+  NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ImportRule, Rule)
-  NS_DECL_ISUPPORTS_INHERITED
-
-  // unhide since nsIDOMCSSImportRule has its own GetStyleSheet and GetMedia
-  using dom::CSSImportRule::GetStyleSheet;
-  using dom::CSSImportRule::GetMedia;
 
   // Rule methods
 #ifdef DEBUG
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
   virtual already_AddRefed<Rule> Clone() const override;
 
   void SetSheet(CSSStyleSheet*);
 
   virtual size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const override;
 
-  // nsIDOMCSSImportRule interface
-  NS_IMETHOD GetHref(nsAString& aHref) final;
-
   // WebIDL interface
   void GetCssTextImpl(nsAString& aCssText) const override;
+  void GetHref(nsAString& aHref) const final;
   dom::MediaList* GetMedia() const final;
   StyleSheet* GetStyleSheet() const final;
 
 private:
   nsString  mURLSpec;
   RefPtr<nsMediaList> mMedia;
   RefPtr<CSSStyleSheet> mChildSheet;
 };

--- a/layout/style/ServoImportRule.cpp
+++ b/layout/style/ServoImportRule.cpp
@@ -91,21 +91,20 @@ ServoImportRule::GetMedia() const
 }
 
 StyleSheet*
 ServoImportRule::GetStyleSheet() const
 {
   return mChildSheet;
 }
 
-NS_IMETHODIMP
-ServoImportRule::GetHref(nsAString& aHref)
+void
+ServoImportRule::GetHref(nsAString& aHref) const
 {
   Servo_ImportRule_GetHref(mRawRule, &aHref);
-  return NS_OK;
 }
 
 /* virtual */ void
 ServoImportRule::GetCssTextImpl(nsAString& aCssText) const
 {
   Servo_ImportRule_GetCssText(mRawRule, &aCssText);
 }
 

--- a/layout/style/ServoImportRule.h
+++ b/layout/style/ServoImportRule.h
@@ -21,31 +21,25 @@ class ServoImportRule final : public dom
 {
 public:
   ServoImportRule(RefPtr<RawServoImportRule> aRawRule,
                   uint32_t aLine, uint32_t aColumn);
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ServoImportRule, dom::CSSImportRule)
 
-  // unhide since nsIDOMCSSImportRule has its own GetStyleSheet and GetMedia
-  using dom::CSSImportRule::GetStyleSheet;
-  using dom::CSSImportRule::GetMedia;
-
 #ifdef DEBUG
   void List(FILE* out = stdout, int32_t aIndent = 0) const final;
 #endif
   already_AddRefed<css::Rule> Clone() const final;
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const final;
 
-  // nsIDOMCSSImportRule interface
-  NS_IMETHOD GetHref(nsAString& aHref) final;
-
   // WebIDL interface
   void GetCssTextImpl(nsAString& aCssText) const override;
+  void GetHref(nsAString& aHref) const final;
   dom::MediaList* GetMedia() const final;
   StyleSheet* GetStyleSheet() const final;
 
 private:
   ~ServoImportRule();
 
   RefPtr<RawServoImportRule> mRawRule;
   RefPtr<ServoStyleSheet> mChildSheet;

--- a/layout/style/nsCSSRules.cpp
+++ b/layout/style/nsCSSRules.cpp
@@ -179,21 +179,20 @@ ImportRule::GetMedia() const
 }
 
 StyleSheet*
 ImportRule::GetStyleSheet() const
 {
   return mChildSheet;
 }
 
-NS_IMETHODIMP
-ImportRule::GetHref(nsAString & aHref)
+void
+ImportRule::GetHref(nsAString& aHref) const
 {
   aHref = mURLSpec;
-  return NS_OK;
 }
 
 /* virtual */ size_t
 ImportRule::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   return aMallocSizeOf(this);
 
   // Measurement of the following members may be added later if DMD finds it is