jquery.ui.datepicker.js (Month picker) in FireFox

jquery.ui.datepicker.js (Month picker) in FireFox

In this library in the function _generateMonthYearHeader why diference with Firefox in the code?
if( ! $.browser.mozilla ){
                html += inst.yearshtml;
                inst.yearshtml = null;
            } else {
                // will be replaced later with inst.yearshtml
                html += '<select class="ui-datepicker-year"><option value="' + drawYear + '" selected="selected">' + drawYear + '</option></select>';
}

if only use month a year select in the witget in firefox generate some irregular status of year select! Some times show only one year!

I trial with and comment the code and all demos are OK
          
           html += inst.yearshtml;
            inst.yearshtml = null;
             
            /*if( ! $.browser.mozilla ){
                html += inst.yearshtml;
                inst.yearshtml = null;
            } else {
                // will be replaced later with inst.yearshtml
                html += '<select class="ui-datepicker-year"><option value="' + drawYear + '" selected="selected">' + drawYear + '</option></select>';
            }*/