Just to clarify: Thunderbird wasn't hacked, your account was. Thunderbird would have no data to assist in that. Your better approach would be to pursue via your online account and email account provider.

I am trying to find out who accessed my account over the Labor Day weekend. Will you help me?

I'm trying to. Any log of whoever logged into your account would at the email provider's site. The hacker would have gone there to access the account.