To clear cookies from one site using Firefox:
1. Click the menu button in the top right of the browser followed by options. Then Select privacy,
2. In the privacy tab, below history there is a drop down menu. Click this and select "Use custom settings for History".
3. This will give you a selection of new options. Select "Show Cookies..." This will give you a list of the cookies linked with the browser. You will be able to use the search bar to navigate large lists.
4. Select the cookie you would like to remove, followed by "Remove Cookies" at the bottom of the tab.
5. You could alternatively "Remove All Cookies" clearing all cookies linked with your browser.