My Thunderbird account

There is no such thing as a 'Thunderbird account'. You do have an account with your email provider, and you use Thunderbird to retrieve mail from the account. Please be more specific about your problem.

Mail to my Thunrderbird account also goes to another account. It's the latter forwarding I'd like to get rid of, and I don't know how.

As stated before, there is no such things as a Thunderbird account. Email accounts are issued by email providers and Thunderbird is email client software and not a provider.

It is possible to create a filter in Thunderbird to forward the incoming email. If that is what you did, remove the filter. The most logical way this is done is at your email providers server. If this is where you did the forwarding, log into your account at your providers web mail page and turn of the forwarding. Only you know how your initiated the forwarding in the first place.