This is an often asked question in Thunderbird support forums, and one that whilst easy to accomplish, is fraught with unexpected consequences.
Why you should be very careful in deleting accounts.
Thunderbird links messages to IMAP accounts, so if your account is IMAP then deleting the account in Thunderbird will simply see all local copies vanish. However if your changing mail providers, as soon as the mail provider removes your account you may well see all the mail in Thunderbird disappear with the server account. That is because the local copies on IMAP mail are more a cache to improve performance that a permanent store.
If however your account is POP and you have deleted the mail from the server when you downloading, your only copy of mail will vanish along with the account. This is the common unexpected consequence of deleting an account, the messages associated with that account will simply vanish. It may be possible to recover these messages, MozillaZine has instructions, but it is certainly not something one would elect to do through choice. So the short of it is that if you have messages you want to keep, you don't really want to delete the account. You might want to disable the account from checking mail, you might also want to remove the outgoing server from Thunderbird, but you don't want to delete the account.
Why you might think your need to delete an account
It is not working, or has stopped working.
This is not Outlook Express where the answer to all problems is delete and recreate, or uninstall and reinstall. Neither approach is likely to fix the vast majority of problems, but is highly likely to loose your mail in the process. If you are having problems with an account that does not work, post your settings, they are shown in Trouble shooting on the help menu at the Thunderbird support site with a clear statement including the actual error messages you are receiving.
You have closed the account with the provider
As mentioned earlier Thunderbird will delete your mail along with the account, so you need to copy the mail to Local folders before Thunderbird can no longer synchronise with the old mail server. Local Folders is local storage not connected with the mail server so when mail is copied there the synchronisation link to the server ends and you have a permanent local copy.