Remove corrupt cookies file

If you cannot log in to websites after removing the cookies associated with it, clearing your Firefox cache, and trying with your extensions and plugins disabled, and have checked security software, you may have a corrupt cookies file in your Firefox profile folder.

This will remove all your stored cookies and essentially log you out of all saved sessions on all websites.

1. Open your profile folder:
   • (Firefox 3.0 and 3.5): See How to find your profile
   1. (Firefox 3.6): At the top of the Firefox window, click on the Help menu and select Troubleshooting Information.... The Troubleshooting Information tab will open.
   2. Under Application Basics, click on Open Containing Folder.
2. From the menu at the top of the Firefox window, select File and then select the Exit menu item.
3. Delete the cookies.sqlite file and any cookies.sqlite-journal files.
4. Restart Firefox.

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