Thunderbird will offer only what the server advertises. If for some silly reason it doesn't announce an available POP capability, you need to insert the required details into the account set-up window. You can just remove the unwanted account. Tools|Account Settings|{select account}|Account Actions->Remove Account This is rather simpler than locating and deleting your entire profile. But there is nothing to stop you having IMAP and POP flavours of the same account, though in the long run this is untidy and unnecessary. My point is that the pre-existing account doesn't prevent you setting up another instance of it, so long as they have different account names.