Try clearing your cache and deleting cookies for the site, that can often fix problems with accessing sites that previously worked. For details on how to do that see Clear cookies and site data in Firefox and How to clear the Firefox cache.

I have tried deleting all cookies and cache several times, but nothing works. I can open Google calendar but when I click on any entry I just get a blank box. If I try to make a new event entry nothing happens. Everything works fine in Internet Explorer.

I am really getting tired of constant problems with Firefox. Every week I have something that doesn't work. One time it's downloading videos, the next week music or email, now Google calendar. I am about ready to give up Firefox forever.