ajax: function( s ) {
    // Extend the settings, but re-extend 's' so that it can be
    // checked again later (in the test suite, specifically)
    s = jQuery.extend(true, s, jQuery.extend(true, {}, jQuery.ajaxSettings, s));
    ...
}

Re-extending 's' seems to conflict with Firefox 3 when doing an ajax post; An unnecessary refresh occurs.

Without the re-extend (like below) it works like a charm in all browsers.

ajax: function( s ) {
    s = jQuery.extend(true, {}, jQuery.ajaxSettings, s));
    ...
}