At firefox 3.6.6 code like $.post("url",somedata,function(){alert('always run')}) always run, even the url server is offline

At firefox 3.6.6 code like $.post("url",somedata,function(){alert('always run')}) always run, even the url server is offline

I want to show some tip when the server is not online.  I have wrote some code like below:

    $(document).ajaxError(function(e,xhr,opt){
        $.pnotify("the ajax is not fine");
    });

Those code is fine in IE. But at Firefox 3.6.6, the handler of  success always be called, even the server has down.


thank u