It is probable that the messages are still present on your computer. Locate your profile and delete the files representing the unwanted account If you really want to cover your tracks, you may also want to use a scrubber utility to overwrite the deleted files.

https://support.mozilla.org/en-US/kb/profiles-tb

Your messages may still also be on a server. Not a "random" server, but the specific email server operated by your email provider that hosted your IMAP account.

The least you should do is visit the account via the email provider's website and delete the unwanted messages.

All of this depends on how thorough you were in attempting to delete messages. Deleting accounts from Thunderbird does not delete the messages themselves. Even if you did delete all the messages, we can't say how your server implemented this deletion. It may have done something similar to Thunderbird, that is, hiding but not deleting unwanted messages. And merely closing the account wouldn't necessarily guarantee that its contents are automatically erased.

Thanks Zenos! I actually took Thunderbird off of my computer, and deleted the email account associated with it. I looked into my files and my profile only had some very small files.

Do you think a scrubber is still necessary to remove this on my computer?

And being that I deleted my email account associated with Thunderbird there's no other location on the web that my emails would be stored right?

Thunderbird doesn't have it's own storage right? It just pulls directly from my emails server?

Thanks for all your help!!

There is no "Thunderbird server". People coming from a webmail experience do tend to expect that Thunderbird is a complete end-to-end service, but no; it is just an email client, a program installed and running on your computer that interacts with the servers operated by email providers. There is no Thunderbird server and there are no Thunderbird accounts.

If you have no further use for Thunderbird then in addition to uninstalling it, you should locate and delete its profile. That will remove stored messages as effectively as as happens when you delete any file. That is, it's invisible but could conceivably be recovered using undelete or forensic recovery utilities.

We can't say what happens on the server. That depends n the server software being used and the email provider's policies. Your account may be deleted and overwritten quite promptly, or your messages may be marked as deleted and remain in place tagged to indicate that the space they occupy is available to be re-used. You'd have to read the small print in the provider's terms and conditions to see what they might actually do.

Again, in broad terms, once you have closed the account, the contents are lost to ordinary users such as you and me. Recovery of the contents of a deleted account would probably involve forensic tools, and a warrant or sub-poena.

Of course, in some legislations, ISPs are required to keep records of your traffic and emails. Perhaps not so much the contents, but meta information, such as about when you sent it and where you sent it to. However, I read that in the US in particular, anything held on a server beyond some modest time limit is automatically available to the relevant authorities. Here in the UK, the list of bodies entitled to search or request such information is alarmingly long and comprehensive; it's not limited just to security and law enforcement.  :-(