--- a/caps/nsJSPrincipals.cpp
+++ b/caps/nsJSPrincipals.cpp
@@ -14,16 +14,40 @@
 #include "nsIJSRuntimeService.h"
 #include "nsIServiceManager.h"
 #include "nsMemory.h"
 #include "nsStringBuffer.h"
 
 // for mozilla::dom::workers::kJSPrincipalsDebugToken
 #include "mozilla/dom/workers/Workers.h"
 
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsJSPrincipals::AddRef()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  NS_PRECONDITION(int32_t(refcount) >= 0, "illegal refcnt");
+  nsrefcnt count = ++refcount;
+  NS_LOG_ADDREF(this, count, "nsJSPrincipals", sizeof(*this));
+  return count;
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsJSPrincipals::Release()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  NS_PRECONDITION(0 != refcount, "dup release");
+  nsrefcnt count = --refcount;
+  NS_LOG_RELEASE(this, count, "nsJSPrincipals");
+  if (count == 0) {
+    delete this;
+  }
+
+  return count;
+}
+
 /* static */ bool
 nsJSPrincipals::Subsume(JSPrincipals *jsprin, JSPrincipals *other)
 {
     bool result;
     nsresult rv = nsJSPrincipals::get(jsprin)->Subsumes(nsJSPrincipals::get(other), &result);
     return NS_SUCCEEDED(rv) && result;
 }
 

--- a/caps/nsJSPrincipals.h
+++ b/caps/nsJSPrincipals.h
@@ -4,51 +4,57 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 /* describes principals by their orginating uris*/
 
 #ifndef nsJSPrincipals_h__
 #define nsJSPrincipals_h__
 #include "jsapi.h"
 #include "nsIPrincipal.h"
 
-struct nsJSPrincipals : nsIPrincipal, JSPrincipals
+class nsJSPrincipals : public nsIPrincipal, public JSPrincipals
 {
+public:
+  /* SpiderMonkey security callbacks. */
   static bool Subsume(JSPrincipals *jsprin, JSPrincipals *other);
   static void Destroy(JSPrincipals *jsprin);
 
   /*
    * Get a weak reference to nsIPrincipal associated with the given JS
-   * principal.
+   * principal, and vice-versa.
    */
   static nsJSPrincipals* get(JSPrincipals *principals) {
     nsJSPrincipals *self = static_cast<nsJSPrincipals *>(principals);
     MOZ_ASSERT_IF(self, self->debugToken == DEBUG_TOKEN);
     return self;
   }
-  
   static nsJSPrincipals* get(nsIPrincipal *principal) {
     nsJSPrincipals *self = static_cast<nsJSPrincipals *>(principal);
     MOZ_ASSERT_IF(self, self->debugToken == DEBUG_TOKEN);
     return self;
   }
 
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
+  NS_IMETHOD_(MozExternalRefCountType) Release(void) override;
+
   nsJSPrincipals() {
     refcount = 0;
     setDebugToken(DEBUG_TOKEN);
   }
 
-  virtual ~nsJSPrincipals() {
-    setDebugToken(0);
-  }
-
   /**
    * Return a string that can be used as JS script filename in error reports.
    */
   virtual void GetScriptLocation(nsACString &aStr) = 0;
 
 #ifdef DEBUG
   virtual void dumpImpl() = 0;
 #endif
 
   static const uint32_t DEBUG_TOKEN = 0x0bf41760;
+
+protected:
+  virtual ~nsJSPrincipals() {
+    setDebugToken(0);
+  }
+
 };
 
 #endif /* nsJSPrincipals_h__ */

--- a/caps/nsNullPrincipal.cpp
+++ b/caps/nsNullPrincipal.cpp
@@ -32,46 +32,16 @@ NS_IMPL_CLASSINFO(nsNullPrincipal, nullp
                   NS_NULLPRINCIPAL_CID)
 NS_IMPL_QUERY_INTERFACE_CI(nsNullPrincipal,
                            nsIPrincipal,
                            nsISerializable)
 NS_IMPL_CI_INTERFACE_GETTER(nsNullPrincipal,
                             nsIPrincipal,
                             nsISerializable)
 
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsNullPrincipal::AddRef()
-{
-  NS_PRECONDITION(int32_t(refcount) >= 0, "illegal refcnt");
-  nsrefcnt count = ++refcount;
-  NS_LOG_ADDREF(this, count, "nsNullPrincipal", sizeof(*this));
-  return count;
-}
-
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsNullPrincipal::Release()
-{
-  NS_PRECONDITION(0 != refcount, "dup release");
-  nsrefcnt count = --refcount;
-  NS_LOG_RELEASE(this, count, "nsNullPrincipal");
-  if (count == 0) {
-    delete this;
-  }
-
-  return count;
-}
-
-nsNullPrincipal::nsNullPrincipal()
-{
-}
-
-nsNullPrincipal::~nsNullPrincipal()
-{
-}
-
 /* static */ already_AddRefed<nsNullPrincipal>
 nsNullPrincipal::CreateWithInheritedAttributes(nsIPrincipal* aInheritFrom)
 {
   nsRefPtr<nsNullPrincipal> nullPrin = new nsNullPrincipal();
   nsresult rv = nullPrin->Init(aInheritFrom->GetAppId(),
                                aInheritFrom->GetIsInBrowserElement());
   return NS_SUCCEEDED(rv) ? nullPrin.forget() : nullptr;
 }

--- a/caps/nsNullPrincipal.h
+++ b/caps/nsNullPrincipal.h
@@ -28,27 +28,21 @@ class nsIURI;
 #define NS_NULLPRINCIPAL_SCHEME "moz-nullprincipal"
 
 class nsNullPrincipal final : public nsJSPrincipals
 {
 public:
   // This should only be used by deserialization, and the factory constructor.
   // Other consumers should use the Create and CreateWithInheritedAttributes
   // methods.
-  nsNullPrincipal();
-
-  // Our refcount is managed by nsJSPrincipals.  Use this macro to avoid an
-  // extra refcount member.
+  nsNullPrincipal() {}
 
-  // FIXME: bug 327245 -- I sorta wish there were a clean way to share the
-  // nsJSPrincipals munging code between the various principal classes without
-  // giving up the NS_DECL_NSIPRINCIPAL goodness.
-  NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_NSIPRINCIPAL
   NS_DECL_NSISERIALIZABLE
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
 
   // Returns null on failure.
   static already_AddRefed<nsNullPrincipal> CreateWithInheritedAttributes(nsIPrincipal *aInheritFrom);
 
   // Returns null on failure.
   static already_AddRefed<nsNullPrincipal>
     Create(uint32_t aAppId = nsIScriptSecurityManager::NO_APP_ID,
            bool aInMozBrowser = false);
@@ -58,17 +52,17 @@ public:
 
   virtual void GetScriptLocation(nsACString &aStr) override;
 
 #ifdef DEBUG
   virtual void dumpImpl() override;
 #endif 
 
  protected:
-  virtual ~nsNullPrincipal();
+  virtual ~nsNullPrincipal() {}
 
   nsCOMPtr<nsIURI> mURI;
   nsCOMPtr<nsIContentSecurityPolicy> mCSP;
   uint32_t mAppId;
   bool mInMozBrowser;
 };
 
 #endif // nsNullPrincipal_h__

--- a/caps/nsPrincipal.cpp
+++ b/caps/nsPrincipal.cpp
@@ -44,47 +44,16 @@ static bool URIIsImmutable(nsIURI* aURI)
     mutableObj &&
     NS_SUCCEEDED(mutableObj->GetMutable(&isMutable)) &&
     !isMutable;
 }
 
 // Static member variables
 const char nsBasePrincipal::sInvalid[] = "Invalid";
 
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsBasePrincipal::AddRef()
-{
-  NS_PRECONDITION(int32_t(refcount) >= 0, "illegal refcnt");
-  // XXXcaa does this need to be threadsafe?  See bug 143559.
-  nsrefcnt count = ++refcount;
-  NS_LOG_ADDREF(this, count, "nsBasePrincipal", sizeof(*this));
-  return count;
-}
-
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsBasePrincipal::Release()
-{
-  NS_PRECONDITION(0 != refcount, "dup release");
-  nsrefcnt count = --refcount;
-  NS_LOG_RELEASE(this, count, "nsBasePrincipal");
-  if (count == 0) {
-    delete this;
-  }
-
-  return count;
-}
-
-nsBasePrincipal::nsBasePrincipal()
-{
-}
-
-nsBasePrincipal::~nsBasePrincipal(void)
-{
-}
-
 NS_IMETHODIMP
 nsBasePrincipal::GetCsp(nsIContentSecurityPolicy** aCsp)
 {
   NS_IF_ADDREF(*aCsp = mCSP);
   return NS_OK;
 }
 
 NS_IMETHODIMP
@@ -111,18 +80,16 @@ void nsPrincipal::dumpImpl()
 NS_IMPL_CLASSINFO(nsPrincipal, nullptr, nsIClassInfo::MAIN_THREAD_ONLY,
                   NS_PRINCIPAL_CID)
 NS_IMPL_QUERY_INTERFACE_CI(nsPrincipal,
                            nsIPrincipal,
                            nsISerializable)
 NS_IMPL_CI_INTERFACE_GETTER(nsPrincipal,
                             nsIPrincipal,
                             nsISerializable)
-NS_IMPL_ADDREF_INHERITED(nsPrincipal, nsBasePrincipal)
-NS_IMPL_RELEASE_INHERITED(nsPrincipal, nsBasePrincipal)
 
 // Called at startup:
 /* static */ void
 nsPrincipal::InitializeStatics()
 {
   Preferences::AddBoolVarCache(
     &gIsWhitelistingTestDomains,
     "layout.css.unprefixing-service.include-test-domains");
@@ -795,18 +762,16 @@ static const char EXPANDED_PRINCIPAL_SPE
 NS_IMPL_CLASSINFO(nsExpandedPrincipal, nullptr, nsIClassInfo::MAIN_THREAD_ONLY,
                   NS_EXPANDEDPRINCIPAL_CID)
 NS_IMPL_QUERY_INTERFACE_CI(nsExpandedPrincipal,
                            nsIPrincipal,
                            nsIExpandedPrincipal)
 NS_IMPL_CI_INTERFACE_GETTER(nsExpandedPrincipal,
                              nsIPrincipal,
                              nsIExpandedPrincipal)
-NS_IMPL_ADDREF_INHERITED(nsExpandedPrincipal, nsBasePrincipal)
-NS_IMPL_RELEASE_INHERITED(nsExpandedPrincipal, nsBasePrincipal)
 
 nsExpandedPrincipal::nsExpandedPrincipal(nsTArray<nsCOMPtr <nsIPrincipal> > &aWhiteList)
 {
   mPrincipals.AppendElements(aWhiteList);
 }
 
 nsExpandedPrincipal::~nsExpandedPrincipal()
 { }

--- a/caps/nsPrincipal.h
+++ b/caps/nsPrincipal.h
@@ -6,31 +6,30 @@
 #ifndef nsPrincipal_h__
 #define nsPrincipal_h__
 
 #include "nsAutoPtr.h"
 #include "nsCOMPtr.h"
 #include "nsJSPrincipals.h"
 #include "nsTArray.h"
 #include "nsAutoPtr.h"
+#include "nsIContentSecurityPolicy.h"
 #include "nsIProtocolHandler.h"
 #include "nsNetUtil.h"
 #include "nsScriptSecurityManager.h"
 
 class nsBasePrincipal : public nsJSPrincipals
 {
 public:
-  nsBasePrincipal();
+  nsBasePrincipal() {}
 
 protected:
-  virtual ~nsBasePrincipal();
+  virtual ~nsBasePrincipal() {}
 
 public:
-  NS_IMETHOD_(MozExternalRefCountType) AddRef(void);
-  NS_IMETHOD_(MozExternalRefCountType) Release(void);
   NS_IMETHOD GetCsp(nsIContentSecurityPolicy** aCsp);
   NS_IMETHOD SetCsp(nsIContentSecurityPolicy* aCsp);
 public:
 
   static const char sInvalid[];
 
 protected:
 
@@ -39,18 +38,18 @@ protected:
 #endif
 
   nsCOMPtr<nsIContentSecurityPolicy> mCSP;
 };
 
 class nsPrincipal final : public nsBasePrincipal
 {
 public:
-  NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_NSISERIALIZABLE
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
   NS_IMETHOD Equals(nsIPrincipal* other, bool* _retval) override;
   NS_IMETHOD EqualsConsideringDomain(nsIPrincipal* other, bool* _retval) override;
   NS_IMETHOD GetHashValue(uint32_t* aHashValue) override;
   NS_IMETHOD GetURI(nsIURI** aURI) override;
   NS_IMETHOD GetDomain(nsIURI** aDomain) override;
   NS_IMETHOD SetDomain(nsIURI* aDomain) override;
   NS_IMETHOD GetOrigin(char** aOrigin) override;
   NS_IMETHOD Subsumes(nsIPrincipal* other, bool* _retval) override;
@@ -128,19 +127,21 @@ class nsExpandedPrincipal : public nsIEx
 {
 public:
   explicit nsExpandedPrincipal(nsTArray< nsCOMPtr<nsIPrincipal> > &aWhiteList);
 
 protected:
   virtual ~nsExpandedPrincipal();
 
 public:
-  NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_NSIEXPANDEDPRINCIPAL
   NS_DECL_NSISERIALIZABLE
+  NS_IMETHODIMP_(MozExternalRefCountType) AddRef() override { return nsJSPrincipals::AddRef(); };
+  NS_IMETHODIMP_(MozExternalRefCountType) Release() override { return nsJSPrincipals::Release(); };
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
   NS_IMETHOD Equals(nsIPrincipal* other, bool* _retval) override;
   NS_IMETHOD EqualsConsideringDomain(nsIPrincipal* other, bool* _retval) override;
   NS_IMETHOD GetHashValue(uint32_t* aHashValue) override;
   NS_IMETHOD GetURI(nsIURI** aURI) override;
   NS_IMETHOD GetDomain(nsIURI** aDomain) override;
   NS_IMETHOD SetDomain(nsIURI* aDomain) override;
   NS_IMETHOD GetOrigin(char** aOrigin) override;
   NS_IMETHOD Subsumes(nsIPrincipal* other, bool* _retval) override;

--- a/caps/nsSystemPrincipal.cpp
+++ b/caps/nsSystemPrincipal.cpp
@@ -24,38 +24,16 @@ NS_IMPL_CLASSINFO(nsSystemPrincipal, nul
                   NS_SYSTEMPRINCIPAL_CID)
 NS_IMPL_QUERY_INTERFACE_CI(nsSystemPrincipal,
                            nsIPrincipal,
                            nsISerializable)
 NS_IMPL_CI_INTERFACE_GETTER(nsSystemPrincipal,
                             nsIPrincipal,
                             nsISerializable)
 
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsSystemPrincipal::AddRef()
-{
-  NS_PRECONDITION(int32_t(refcount) >= 0, "illegal refcnt");
-  nsrefcnt count = ++refcount;
-  NS_LOG_ADDREF(this, count, "nsSystemPrincipal", sizeof(*this));
-  return count;
-}
-
-NS_IMETHODIMP_(MozExternalRefCountType)
-nsSystemPrincipal::Release()
-{
-  NS_PRECONDITION(0 != refcount, "dup release");
-  nsrefcnt count = --refcount;
-  NS_LOG_RELEASE(this, count, "nsSystemPrincipal");
-  if (count == 0) {
-    delete this;
-  }
-
-  return count;
-}
-
 static const char SYSTEM_PRINCIPAL_SPEC[] = "[System Principal]";
 
 void
 nsSystemPrincipal::GetScriptLocation(nsACString &aStr)
 {
     aStr.Assign(SYSTEM_PRINCIPAL_SPEC);
 }
 
@@ -220,20 +198,8 @@ nsSystemPrincipal::Read(nsIObjectInputSt
 }
 
 NS_IMETHODIMP
 nsSystemPrincipal::Write(nsIObjectOutputStream* aStream)
 {
     // no-op: CID is sufficient to identify the mSystemPrincipal singleton
     return NS_OK;
 }
-
-/////////////////////////////////////////////
-// Constructor, Destructor, initialization //
-/////////////////////////////////////////////
-
-nsSystemPrincipal::nsSystemPrincipal()
-{
-}
-
-nsSystemPrincipal::~nsSystemPrincipal()
-{
-}

--- a/caps/nsSystemPrincipal.h
+++ b/caps/nsSystemPrincipal.h
@@ -15,30 +15,25 @@
 { 0x4a6212db, 0xaccb, 0x11d3, \
 { 0xb7, 0x65, 0x0, 0x60, 0xb0, 0xb6, 0xce, 0xcb }}
 #define NS_SYSTEMPRINCIPAL_CONTRACTID "@mozilla.org/systemprincipal;1"
 
 
 class nsSystemPrincipal final : public nsJSPrincipals
 {
 public:
-    // Our refcount is managed by nsJSPrincipals.  Use this macro to avoid
-    // an extra refcount member.
-    NS_DECL_ISUPPORTS_INHERITED
     NS_DECL_NSIPRINCIPAL
     NS_DECL_NSISERIALIZABLE
+    NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
 
-    nsSystemPrincipal();
+    nsSystemPrincipal() {}
 
     virtual void GetScriptLocation(nsACString &aStr) override;
 
 #ifdef DEBUG
     virtual void dumpImpl() override;
 #endif 
 
 protected:
-    virtual ~nsSystemPrincipal(void);
-
-    // XXX Probably unnecessary.  See bug 143559.
-    NS_DECL_OWNINGTHREAD
+    virtual ~nsSystemPrincipal(void) {}
 };
 
 #endif // nsSystemPrincipal_h__