diff --git a/embedding/components/windowwatcher/src/nsPromptService.cpp b/embedding/components/windowwatcher/src/nsPromptService.cpp
index 413316e1594..372fd9b20e7 100644
--- a/embedding/components/windowwatcher/src/nsPromptService.cpp
+++ b/embedding/components/windowwatcher/src/nsPromptService.cpp
@@ -58,7 +58,6 @@ static const char kPromptURL[] = "chrome://global/content/commonDialog.xul";
 static const char kSelectPromptURL[] = "chrome://global/content/selectDialog.xul";
 static const char kQuestionIconClass[] = "question-icon";
 static const char kAlertIconClass[] = "alert-icon";
-static const char kWarningIconClass[] = "message-icon";
 // We include question-icon for backwards compatibility
 static const char kAuthenticationIconClass[] = "authentication-icon question-icon";
 
diff --git a/toolkit/content/commonDialog.js b/toolkit/content/commonDialog.js
index 60abef0d3e6..2353a2b7bad 100644
--- a/toolkit/content/commonDialog.js
+++ b/toolkit/content/commonDialog.js
@@ -166,9 +166,8 @@ function commonDialogOnLoad()
   // set the icon
   var iconElement = document.getElementById("info.icon");
   var iconClass = gCommonDialogParam.GetString(2);
-  if (!iconClass)
-    iconClass = "message-icon";
-  iconElement.setAttribute("class", iconElement.getAttribute("class") + " " + iconClass);
+  if (iconClass)
+    iconElement.classList.add(iconClass);
 
   switch (nButtons) {
     case 4: