If you want to import Zimbra emails to Thunderbird without losing attachments, the easiest way is to convert the Zimbra TGZ mailbox into a format supported by Thunderbird, such as MBOX. Thunderbird works smoothly with MBOX files, so once the conversion is complete, you can simply import the file into Thunderbird and access all emails along with their attachments. Many users use tools like SysInfo Zimbra Converter to perform this conversion because it can export TGZ files to MBOX while keeping the original emails and attachments intact. After the conversion, you only need to import the generated MBOX file into Thunderbird to view all the messages without any missing files.
Read more: https://www.sysinfotools.com/blog/import-zimbra-to-thunderbird/