Focus OnBlur In Firefox?

I have a problem where there is an onBlur check for a valid entry. The javascript checks the entry and if it is incorrect it does an alert and then puts the focus back onto the incorrect field, so valid data has to be entered.

This is a generic function so it uses 'this' - onBlur="chkMonthDob(this);"

The function then checks it and puts the focus back on 'this' if it is wrong. This all works fine in IE but in Firefox it does not put the focus back.

Using the Firefox getaround :- setTimeout("document.orgsp.d1_org_dob_mth.focus()", 1); works, however, when it is called with 'this' as a parameter you don't know what the field is to be focussed on, so I had hoped that putting :-

var focusField = myField.id;
setTimeout("document.getElementById(focusField).focus()", 1);

would work - but Firefox say focusField is undefined.