--- a/caps/BasePrincipal.h
+++ b/caps/BasePrincipal.h
@@ -35,16 +35,29 @@ public:
     eNullPrincipal,
     eCodebasePrincipal,
     eExpandedPrincipal,
     eSystemPrincipal
   };
 
   explicit BasePrincipal(PrincipalKind aKind);
 
+  template<typename T>
+  bool Is() const
+  {
+    return mKind == T::Kind();
+  }
+
+  template<typename T>
+  T* As()
+  {
+    MOZ_ASSERT(Is<T>());
+    return static_cast<T*>(this);
+  }
+
   enum DocumentDomainConsideration { DontConsiderDocumentDomain, ConsiderDocumentDomain};
   bool Subsumes(nsIPrincipal* aOther, DocumentDomainConsideration aConsideration);
 
   NS_IMETHOD GetOrigin(nsACString& aOrigin) final;
   NS_IMETHOD GetOriginNoSuffix(nsACString& aOrigin) final;
   NS_IMETHOD Equals(nsIPrincipal* other, bool* _retval) final;
   NS_IMETHOD EqualsConsideringDomain(nsIPrincipal* other, bool* _retval) final;
   NS_IMETHOD Subsumes(nsIPrincipal* other, bool* _retval) final;

--- a/caps/ContentPrincipal.h
+++ b/caps/ContentPrincipal.h
@@ -25,16 +25,18 @@ public:
   NS_IMETHOD GetDomain(nsIURI** aDomain) override;
   NS_IMETHOD SetDomain(nsIURI* aDomain) override;
   NS_IMETHOD GetBaseDomain(nsACString& aBaseDomain) override;
   NS_IMETHOD GetAddonId(nsAString& aAddonId) override;
   bool IsCodebasePrincipal() const override { return true; }
 
   ContentPrincipal();
 
+  static PrincipalKind Kind() { return eCodebasePrincipal; }
+
   // Init() must be called before the principal is in a usable state.
   nsresult Init(nsIURI* aCodebase,
                 const mozilla::OriginAttributes& aOriginAttributes,
                 const nsACString& aOriginNoSuffix);
 
   virtual nsresult GetScriptLocation(nsACString& aStr) override;
 
   /**

--- a/caps/ExpandedPrincipal.h
+++ b/caps/ExpandedPrincipal.h
@@ -15,16 +15,18 @@
 class ExpandedPrincipal : public nsIExpandedPrincipal
                         , public mozilla::BasePrincipal
 {
 public:
   static already_AddRefed<ExpandedPrincipal>
   Create(nsTArray<nsCOMPtr<nsIPrincipal>>& aWhiteList,
          const mozilla::OriginAttributes& aAttrs);
 
+  static PrincipalKind Kind() { return eExpandedPrincipal; }
+
   NS_DECL_NSIEXPANDEDPRINCIPAL
   NS_DECL_NSISERIALIZABLE
 
   NS_IMETHOD_(MozExternalRefCountType) AddRef() override { return nsJSPrincipals::AddRef(); };
   NS_IMETHOD_(MozExternalRefCountType) Release() override { return nsJSPrincipals::Release(); };
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
   NS_IMETHOD GetHashValue(uint32_t* aHashValue) override;
   NS_IMETHOD GetURI(nsIURI** aURI) override;

--- a/caps/NullPrincipal.h
+++ b/caps/NullPrincipal.h
@@ -36,16 +36,18 @@ public:
   // This should only be used by deserialization, and the factory constructor.
   // Other consumers should use the Create and CreateWithInheritedAttributes
   // methods.
   NullPrincipal()
     : BasePrincipal(eNullPrincipal)
   {
   }
 
+  static PrincipalKind Kind() { return eNullPrincipal; }
+
   NS_DECL_NSISERIALIZABLE
 
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
   NS_IMETHOD GetHashValue(uint32_t* aHashValue) override;
   NS_IMETHOD SetCsp(nsIContentSecurityPolicy* aCsp) override;
   NS_IMETHOD GetURI(nsIURI** aURI) override;
   NS_IMETHOD GetDomain(nsIURI** aDomain) override;
   NS_IMETHOD SetDomain(nsIURI* aDomain) override;

--- a/caps/SystemPrincipal.h
+++ b/caps/SystemPrincipal.h
@@ -25,16 +25,18 @@ class SystemPrincipal final : public moz
   SystemPrincipal()
     : BasePrincipal(eSystemPrincipal)
   {
   }
 
 public:
   static already_AddRefed<SystemPrincipal> Create();
 
+  static PrincipalKind Kind() { return eSystemPrincipal; }
+
   NS_DECL_NSISERIALIZABLE
   NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) 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 GetCsp(nsIContentSecurityPolicy** aCsp) override;
   NS_IMETHOD SetCsp(nsIContentSecurityPolicy* aCsp) override;