--- a/dom/base/PostMessageEvent.cpp
+++ b/dom/base/PostMessageEvent.cpp
@@ -103,20 +103,32 @@ PostMessageEvent::Run()
       return NS_OK;
 
     // Note: This is contrary to the spec with respect to file: URLs, which
     //       the spec groups into a single origin, but given we intentionally
     //       don't do that in other places it seems better to hold the line for
     //       now.  Long-term, we want HTML5 to address this so that we can
     //       be compliant while being safer.
     if (!targetPrin->Equals(mProvidedPrincipal)) {
-      MOZ_DIAGNOSTIC_ASSERT(ChromeUtils::IsOriginAttributesEqualIgnoringFPD(mProvidedPrincipal->OriginAttributesRef(),
-                                                                            targetPrin->OriginAttributesRef()),
-                            "Unexpected postMessage call to a window with mismatched "
-                            "origin attributes");
+      OriginAttributes sourceAttrs = mProvidedPrincipal->OriginAttributesRef();
+      OriginAttributes targetAttrs = targetPrin->OriginAttributesRef();
+
+      MOZ_DIAGNOSTIC_ASSERT(sourceAttrs.mAppId == targetAttrs.mAppId,
+        "Target and source should have the same mAppId attribute.");
+      MOZ_DIAGNOSTIC_ASSERT(sourceAttrs.mUserContextId == targetAttrs.mUserContextId,
+        "Target and source should have the same userContextId attribute.");
+      MOZ_DIAGNOSTIC_ASSERT(sourceAttrs.mInIsolatedMozBrowser == targetAttrs.mInIsolatedMozBrowser,
+        "Target and source should have the same inIsolatedMozBrowser attribute.");
+
+      if (!nsContentUtils::IsSystemOrExpandedPrincipal(targetPrin) &&
+          !nsContentUtils::IsSystemOrExpandedPrincipal(mProvidedPrincipal) &&
+          !mTrustedCaller) {
+        MOZ_DIAGNOSTIC_ASSERT(sourceAttrs.mPrivateBrowsingId == targetAttrs.mPrivateBrowsingId,
+          "Target and source should have the same mPrivateBrowsingId attribute.");
+      }
 
       nsAutoString providedOrigin, targetOrigin;
       nsresult rv = nsContentUtils::GetUTFOrigin(targetPrin, targetOrigin);
       NS_ENSURE_SUCCESS(rv, rv);
       rv = nsContentUtils::GetUTFOrigin(mProvidedPrincipal, providedOrigin);
       NS_ENSURE_SUCCESS(rv, rv);
 
       const char16_t* params[] = { providedOrigin.get(), targetOrigin.get() };