DatePicker year select dropdown has only one entry with multiple months showing

Ref: http://forum.jquery.com/topic/datepicker-year-select-dropdown-has-only-one-entry-with-multiple-months-showing.

This has been caused by the deferred update of the year drop-down to avoid flashes on Firefox.

Solution: wrap the generation of the ''inst.yearshtml'' property in ''_generateMonthYearHeader'' to only do it once.

		// year selection
		if (!inst.yearshtml) {
			inst.yearshtml = '';
			...
				//when showing there is no need for later update
				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>';
				}
			}
		}