What is the account type of the account to be deleted - POP or IMAP?

It's a POP account

With POP it's configurable whether to leave messages on the server or not. In the latter case, you may have downloaded messages not present in the account to be kept. In that case you may want to backup those, or just copy them to the account to be kept. Otherwise you can safely delete the POP account.

All the messages I want to store are already in the account (let's call it Account A) to be kept. The other account (let's call it Account B, which has the same email address) is the one I now want to delete. My concern is that by deleting Account B, I'll lose all the emails stored in Account A.

POP accounts are independent of each other. So you can safely delete Account B, without impacting A. Having said that, it's always recommended to create a full backup of the Thunderbird profile prior to making changes. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

OK, that worked, thanks! I have another Thunderbird Mozilla email issue I'll be dealing with over the next few days. Will you be available to help with that one, too?