I am have trouble using the calendar in firefox, it works perfectly fine in IE but when using FF3 I get the following error

"dateEl is null"
cal.parseDate(dateEl.value || dateEl.innerHTML);

This happens when I click the calendar icon, the calendar does not appear, just that error (picked up with firebug, if firebug is not used then nothing happens at all)

Is this a known issue at all? any ideas? Thanks very much