Will uninstalling Thunderbird from my computer also delete all the emails downloaded to my computer?

No, messages and all your other data (contacts, filters, etc.) are stored separately from the program installation in the Thunderbird profile.

If not, how can I delete the email messages that are stored offline somewhere on my computer?

Delete the profile folder. Note, that will remove all your data, not just the messages. To locate the Thunderbird profile see https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Thanks for your response. Just to be sure I understand correctly: - after deleting the profile folder everything that Thunderbird has stored on my computer (including contacts and messages) will be removed. But all my contacts and messages on the server itself and in other mail clients that I use for the same email account will be kept intact, is that correct?

- after deleting the profile folder everything that Thunderbird has stored on my computer (including contacts and messages) will be removed.

Yes.

But all my contacts and messages on the server itself and in other mail clients that I use for the same email account will be kept intact, is that correct?

Correct.

Thank you! :)