how to set default button if there are more than one submit buttons in Firefox

Hi friends my form contains two submit buttons one is for back button
and another is for submit button when the user press enter in any textbox
the submit button will be .I wrote a javascript its working fine in IE
but not in FireFox please help me

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document.onkeyup = KeyCheck;
function KeyCheck(e)
{
var KeyID = (window.event) ? event.keyCode : e.keyCode;
if(KeyID==13)
{


document.getElementById("ff").disabled=true;
document.getElementById("ss").focus();
document.getElementById("ss").click();
}
}