--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -1784,16 +1784,24 @@ nsStandardURL::SetUsername(const nsACStr
 
 nsresult
 nsStandardURL::SetPassword(const nsACString &input)
 {
     ENSURE_MUTABLE();
 
     const nsPromiseFlatCString &password = PromiseFlatCString(input);
 
+    auto clearedPassword = MakeScopeExit([&password, this]() {
+        // Check that if this method is called with the empty string then the
+        // password is definitely cleared when exiting this method.
+        if (password.IsEmpty()) {
+            MOZ_DIAGNOSTIC_ASSERT(this->Password().IsEmpty());
+        }
+    });
+
     LOG(("nsStandardURL::SetPassword [password=%s]\n", password.get()));
 
     if (mURLType == URLTYPE_NO_AUTHORITY) {
         if (password.IsEmpty())
             return NS_OK;
         NS_WARNING("cannot set password on no-auth url");
         return NS_ERROR_UNEXPECTED;
     }