javascript and asp dot net

[asoft2008]

Hi ever body

I have just created a gridview and on click event i want to open information at bottom existing iframe. I have follow this code:

javascript.
function showMessage(id)
{
var ids = id;
//frames['showPage'].location.href='welcome.aspx?messageid=' + id;
document.getElementById('myMessage').location.href =id;
}

C# Code

e.Row.Cells[1].Attributes.Add("onclick", "showMessage('readMsg.aspx?messageid=" + e.Row.Cells[1].Text.ToString() + "')");

it working 100% on IE but Fire fox it does not do any thing and i got an error

document.getElementById("myMessage") has no properties

have any idea about this Thanks