How do I prevent Thunderbird from pulling in hundreds of emails that I had previously deleted in my gmail account?

I want to use gmail to initially view my emails, delete many, and then just pull in to Thunderbird those emails I want to save. However, when I ask Thunderbird to "get messages," it pulls in up to 250 emails, most of which I had deleted from gmail days or weeks ago. I would like Thunderbird to get only those emails that I have not deleted from gmail. Help!!!