Short answer is you can not. There are ways to get the messages into local folders so you can delete the account. But you can not disable an account.

Matt thank you. Is there any way to ask Mozilla to add this feature? It would be trivial to do so, and many mail clients allow you to select which of the email accounts should automatically check for new mail.

Even if you uncheck all the boxes in Tools/Account Settings, Server Settings/Server Settings for the account, the account may still download new mail if the server supports the IDLE command, i.e. notifications are automatically sent when new mail arrives. Although I've never tried it, unsubscribing to Inbox should leave the account in place, but stop new mail from being downloaded.