duplicate web-mail addresses in BT Mail

My ISP is BT (UK) and I have 5 BT Mail email addresses. On the mail log-in page there is a sign-in box where I can choose which email address I wish to open. Unfortunately there are several instances of my main email address. Some with old passwords some from I know not where. I need to delete all those except the current one. I have tried clearing the cache and data but to no effect. The duplicates do not appear if I use a different browser, so it would seem to be a Firefox issue.