diff --git a/js/xpconnect/src/XPCWrappedJSClass.cpp b/js/xpconnect/src/XPCWrappedJSClass.cpp
index 970d4bb0dab..0c563f4da6f 100644
--- a/js/xpconnect/src/XPCWrappedJSClass.cpp
+++ b/js/xpconnect/src/XPCWrappedJSClass.cpp
@@ -573,6 +573,22 @@ nsXPCWrappedJSClass::DelegatedQueryInterface(nsXPCWrappedJS* self,
                                              REFNSIID aIID,
                                              void** aInstancePtr)
 {
+    if (MOZ_UNLIKELY(!NS_IsMainThread())) {
+        printf("Uh oh! DelegatedQueryInterface called off-main-thread!\n");
+        printf("Name: %s\n", GetInterfaceName());
+        JSCompartment *c = js::GetObjectCompartment(self->GetJSObjectPreserveColor());
+        char *origin = nullptr;
+        nsresult rv = xpc::GetCompartmentPrincipal(c)->GetOrigin(&origin);
+        if (NS_SUCCEEDED(rv)) {
+            printf("Principal origin: %s\n", origin);
+            NS_Free(origin);
+        } else {
+            printf("Unable to get origin from principal :-(\n");
+        }
+        nsAutoCString loc(EnsureCompartmentPrivate(c)->GetLocation());
+        printf("Global's Location: %s\n", loc.get());
+        MOZ_CRASH();
+    }
     if (aIID.Equals(NS_GET_IID(nsIXPConnectJSObjectHolder))) {
         NS_ADDREF(self);
         *aInstancePtr = (void*) static_cast<nsIXPConnectJSObjectHolder*>(self);