Cleaning your computer and cleaning your cookies are an entirely different thing.
Go to Firefox.
Select Tools -> options
On the Privacy tab, there is a button marked "Show Cookies". CLick that
In the panel that opens up, there at the top is a "Search" field. Enter "thenext" (without the quotes).
what will show up are just the cookies related to this site.
select ONE cookie and click the "Remove Cookie" button. DO NOT CLICK THE "REMOVE ALL COOKIES" BUTTON.
When all the cookies for this site are removed, close (shut down) your browser (Firefox) and restart it. Come to this site and see if your problem has gone away.
~Tom