Form1.submit(); In Firefox? Works In IE

I have this little nice part, which works in IE, but not in Firefox.
Can anyone help me?

It seems that one gets this error: form1.elements is not a function

What should I use instead?

function directclick(input)
{
pos=input.indexOf("_");
form1 = document.forms[1];
form1.elements("aaa").value=input.substring(0,pos);
form1.elements("bbb").value=input.substring(pos+1);
form1.submit();
// this is not automatically called :(
DisableSubmits(); // my own func which disables submit buttons
}