Thank to all for kind responses.
Follow your instruction, I have used the Firefox Javascript Console. I do have a better clue now. But, it is not enough for me figure out the problem.
I have tried:
document.forms[0].sortKey.value = "firstName";
and
document.FindUsersForm.sortKey.value = "firstName";
The Firefox Javascript Console indicated that both of the two statements above have no property. That is:
document.forms[0].sortKey has no properties.
and
document.FindUsersForm.sortKey has no properties;
I have triple checked my FindUsersForm. It defines
String SortKey and String orderKey. Both of them have proper getter and setters.
I have also 'view source' of the displayed web page. But, if offers no clue because the clicking on the link does not work at all.
Thanks in advance.