According to their support page you start with the menu at the top of the page.

Delete an email account

Menu location: Tools | Account Settings | Account Actions

From there you should be presented with options including removing or deleting the account.

Those instructions are from about 2/3 the way down this page:

http://support.mozillamessaging.com/en-US/kb/configuration-options-accounts
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"There is not a man of us who does not at times need a helping hand to be stretched out to him, and then shame upon him who will not stretch out the helping hand to his brother." -Theodore Roosevelt