If you want to continue to be able to read the 'comcast' emails, you would need to move all emails from all of the pop comcast folders into the 'Local Folders' mail account before removing the pop comcast mail account.

I don't understand what you mean by "Local Folders mail account" All the Comcast pop mail folders (Inbox, Drafts, Templates, Sent, Junk, Trash, and Outbox) are in the Local Folders main folder. I also created some additional folders within Local Folders that I would like to keep.

Thunderbird comes with a built-in Local Folders account, and you add new accounts. So the basic starting point is Local Folders and at least one "real" account. If you have a single account then you have done something out of the ordinary, such as renaming Local Folders, or maybe you have set a POP account to use the Global Inbox. It would be helpful, to me at least, to unravel this before doing much more.

An account that uses POP stores its messages locally, in Thunderbird, so if the operator closed down tomorrow, you wouldn't lose anything. So there's no need to panic. You would however still have an active account in Thunderbird that would try, fruitlessly, to contact the now non-existent server and harangue you with error messages. This is a reason for moving these messages to a place that wouldn't look for a connection to a server, which is the usual reason for moving messages to Local Folders.

Alternatively, if you have chosen and set up a new email account with an alternative provider, you could put your old messages into this new account.

Either way, once you have rescued all the messages you want to keep, you can remove the old account and avoid all the error messages when it doesn't connect. If you are using the Global Inbox, then you effectively already have everything you want to keep in Local Folders.

And if you want to continue using the Global Inbox, rather than having separate accounts with their own Inboxes etc, take care to set up your new account using POP.