Click Options in the top menu
Select Privacy in the Options tabs
Next, there are two steps to this. Firefox has TWO places where you must clear the cookies, or it will not work.
Removing cookies -
You have two ways to go about this. Remove them individually, or remove them all at the same time.
To remove individually, and be sure you get ALL adultfanfiction.net cookies
In the search box, type in .adultfanfiction.net. This will bring up all AFF cookies
Remove each cookie that shows up in the list with that search parameter
To remove all cookies - select "Remove all Cookies"
The last step is to clear from the history link
Make sure the time range selected is "Everything"
You must select Cookies, Cache, and Active Logins.
Click, "Clear Now"
Close the browser, and restart it.