- click 'account settings' - in left sidebar, highlight one of the accounts, scroll down slightly to see the 'Account Actions' menu on same side of screen. Click that an select 'remove account. That will include a box to check if you want messages to also be deleted. - repeat for the other account.

It won't let me remove the account, the "remove account" text is faded. It will let me remove accounts that I want to keep!

Puzzling. See if this works and let me know: https://support.mozilla.org/en-US/questions/1344354

It won't let me remove the account, the "remove account" text is faded. It will let me remove accounts that I want to keep!

The Default account can't be removed. Set another account as Default, with Account Actions, then remove the first account.