Sorry, you don't have (and never had) a "Thunderbird account", unless, maybe, you had a formal position with Mozilla as a developer or team member (but then you shouldn't have used such an address for non-mozilla activities). Thunderbird/Mozilla doesn't supply email accounts to the general public. No-one at Mozilla would have any record of any of your personal email accounts. You need to discover who your email provider was, but it's unlikely they could or would be able to help you if the account has been idle for any great length of time. If you are still using the same computer (unlikely!) and it hasn't been vigorously wiped clean then if you reinstallled Thunderbird it might pick up the old profile, and show your account particulars.