Go here to assist you with this issue:

http://pcsupport.about.com/od/maintenan ... acheff.htm

It doesn't give step-by-step for deleting cookies but from the screenshots you can figure it out.

Removing Cookies

Important: To remove cookies, follow the steps in this section. Do not try to edit the cookies file on your computer.

To remove one or more cookies from your computer:

1. Open the Tasks menu, choose Privacy & Security, and then choose Cookie Manager.
2. Choose View Stored Cookies from the submenu. The Cookie Manager window opens with a list of all the cookies stored on your computer.
3. Select one or more cookies and click Remove Cookie, or click Remove All Cookies.

You can also choose to prevent the removed cookies from being re-accepted later.


This is a good link that describes in detail everything you can do with cookie manager in FireFox suggest you read it

http://www.mozilla.org/projects/securit ... _help.html