Thunderbird stores its emails in MBOX format, so it's very basic to convert them to HTML. You might try TrustVare MBOX to HTML Converter - it is a good program to do this with little Efforts. Just load your MBOX file, choose HTML as the output format, and let it do its thing. It preserves attachments, formatting, and folder structure too. For large mailboxes, TrustVare honestly saves a ton of time. Hope this helps!