Problem with AJAX only on Mozilla

Hello guys,

I try to implement a chat application using PHP + MySql. The problem so far is that when I am chatting, I "lose" inputs when a small part of the page goes to be updated. On the other web browsers (Chrome, IE, Opera) I have some delay but at least the words are correct. I think the problem is with the

setTimeout("showMessages()",3000);

but I don't know how to improve the source code or if I have to use something else.
I try to make it as reliable as possible. I don't want to increase the refresh time because when a user "says" something, then the others must be able to see it almost simultaneously.
Any idea please??

Here is the source code:

function keyEvent(e)
        {
            if (window.event)
            {
                e = window.event;
            }
            if (e.keyCode==13)
            {
              send();
            }
        }


function getObject()
{
 if (window.ActiveXObject) return new ActiveXObject("Microsoft.XMLHTTP");
    else if (window.XMLHttpRequest) return new XMLHttpRequest();
    else {
    alert("Ajax is not supported, try a different browser.");
    return null;
    }
}

function showMessages()
{
    httpObject = getObject();
    httpObject.open("GET", "apath/show-messages.php?", false);// + Math.random(), false); 
    httpObject.send(null);
    document.getElementById("messages").innerHTML = httpObject.responseText;
    setTimeout("showMessages()",3000); //refresh the messages every 3 seconds
   
}

showMessages(); 

function send()
{
    httpObject = getObject();
    httpObject.open("GET", "apath/send.php?message=" + document.getElementById('message').value, false);
    httpObject.send(null);
    document.getElementById("message").value = "";
}