No idea. Firefox does not come with a Calendar. You have not named the calendar. Mozilla ok's Extensions submitted by their Developers to work with Firefox and provide a Web Page for their distribution. Please go to the Developers home page of the calendar and check to see what is going on or look for a alternative. A small desktop one is Rainlender http://www.rainlendar.net/cms/index.php https://support.mozilla.org/en-US/kb/firefox-add-technology-modernizing Please let us know if this solved your issue or if need further assistance.