Clear all cookies and the cache

If deleting cookies for the site with the problem did not fix the error, clear all the cookies stored on your computer and clear the Firefox cache:

1. On the menu bar, click Tools, then select Clear Recent History....
2. In the Time Range to clear: dropdown, select Everything.
3. Click the arrow next to Details to display the list of items that can be cleared. 
    
4. Select both Cookies and Cache.
5. Click Clear Now.

Help by http://support.mozilla.com/en-US/kb/Cannot%20log%20in%20to%20websites?s=delete+cookies&as=s