For example,
Error: [Exception... "'Error: No menu commaner found for URL: http://idwhiz.vicp.net/' when calling method: [nsIDOMEventListener::handleEvent]"  nsresult: "0x8057001c (NS_ERROR_XPC_JS_THREW_JS_OBJECT)"  location: "<unknown>"  data: no]
Maybe you installed the GreaseMonkey extension but you disabled it on someday. Re-enable the GreaseMonkey will remove these error message from JavaScript console.