diff --git a/caps/BasePrincipal.cpp b/caps/BasePrincipal.cpp
index cdd4665fc8..ade5b4e2e0 100644
--- a/caps/BasePrincipal.cpp
+++ b/caps/BasePrincipal.cpp
@@ -560,6 +560,23 @@ BasePrincipal::GetCspJSON(nsAString& outCSPinJSON)
   return mCSP->ToJSON(outCSPinJSON);
 }
 
+NS_IMETHODIMP
+BasePrincipal::IsSameOrigin(nsIURI* aURI, bool aIsPrivateWin, bool* aRes) {
+  *aRes = false;
+  nsCOMPtr<nsIURI> prinURI;
+  nsresult rv = GetURI(getter_AddRefs(prinURI));
+  if (NS_FAILED(rv) || !prinURI) {
+    return NS_OK;
+  }
+  nsIScriptSecurityManager* ssm = nsContentUtils::GetSecurityManager();
+  if (!ssm) {
+    return NS_ERROR_UNEXPECTED;
+    ;
+  }
+  *aRes = NS_SUCCEEDED(ssm->CheckSameOriginURI(prinURI, aURI, aIsPrivateWin));
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 BasePrincipal::GetIsNullPrincipal(bool* aResult)
 {
diff --git a/caps/BasePrincipal.h b/caps/BasePrincipal.h
index b1f151521f..dd60d33746 100644
--- a/caps/BasePrincipal.h
+++ b/caps/BasePrincipal.h
@@ -282,6 +282,7 @@ public:
   NS_IMETHOD GetUnknownAppId(bool* aUnknownAppId) final;
   NS_IMETHOD GetUserContextId(uint32_t* aUserContextId) final;
   NS_IMETHOD GetPrivateBrowsingId(uint32_t* aPrivateBrowsingId) final;
+  NS_IMETHOD IsSameOrigin(nsIURI* aURI, bool aIsPrivateWin, bool* aRes) override;
 
   bool EqualsIgnoringAddonId(nsIPrincipal *aOther);
 
diff --git a/caps/nsIPrincipal.idl b/caps/nsIPrincipal.idl
index 1d58dcb7fa..03f293f7b2 100644
--- a/caps/nsIPrincipal.idl
+++ b/caps/nsIPrincipal.idl
@@ -223,6 +223,12 @@ interface nsIPrincipal : nsISerializable
      */
     readonly attribute AUTF8String originSuffix;
 
+    /**
+     * Uses NS_Security Compare to determine if the
+     * other URI is same-origin as the uri of the Principal
+     */
+    bool IsSameOrigin(in nsIURI otherURI, in bool aIsPrivateWin);
+
     /**
      * The base domain of the codebase URI to which this principal pertains
      * (generally the document URI), handling null principals and