If you want to migrate Thunderbird MBOX files to Gmail without data loss, using a dedicated DRS Softech MBOX Converter Tool migration solution can make the process safer and much easier. Thunderbird stores mailbox data in MBOX format, but Gmail does not support direct MBOX file importing. It also imports MBOX to Gmail. https://www.drssoftech.com/blog/import-mbox-to-gmail-without-thunderbird/