If you want to easily migrate all of your Thunderbird emails, the Softaken Thunderbird Converter Suite can do that. Since it lets you download emails in a batch, you don't have to do it one at a time. You can send your entire Thunderbird email in one go. It allows you to export or turn into many formats, such as PST, EML, MSG, EMLX, HTML, MHTML, and others, depending on what you need. It saves your files just like they are, with headers for emails and attachments.

I have been using Mozilla Thunderbird for several years.

If your account is an IMAP account then imapsync can help you to backup it to another IMAP account. The format is irrelevant in that case.
If not, well it's not the best place to ask. Thunderbird https://github.com/thunderbird might be a better place.

You can safely migrate Thunderbird emails to other formats by using an IMAP-based migration tool that preserves data integrity and folder structure. I have used the EdbMails IMAP Migration Tool, and it worked well for migrating Thunderbird mailboxes without manual export or complicated setup.

Since Thunderbird supports IMAP, EdbMails connects directly to the account and migrates data to destinations like PST, Office 365, Exchange Server, or other IMAP-supported platforms. During my use, it maintained the original folder structure and message properties, and the selective migration options helped move only the required data.