Are the accounts set up as POP or IMAP in Thunderbird... most important with regard to the next steps.

According to our work tech support it is IMAP

One idea: You could copy emails from imap folder to a new folder in 'Local Folders' mail account. Suggest you do it in batches and switch off any email scanning to help speed up process.

If you go into 'offline' mode and synchronise folders to ensure you have full download in the imap folders, then copy emails to 'Local Folders' in batches,then check you can read emails in the 'Local Folders' mail account. If yes, then you now have a copy which is stored on your computer in the Thunderbird Profile folder.

Once all emails are copied over to 'Local Folders' mail account, then you can choose to remove the imap account if required.

It would your responsibility to create backups of your Profile name folder.

Thanks, but I understand I will still will need a personal Thunderbird account to be able to access emails saved to local folders. If my current employer discontinues Thunderbird, how can I set up a separate account? The domain would need to be different.

re :If my current employer discontinues Thunderbird, how can I set up a separate account? I'm a bit confused with this comment...... Thunderbird is just an email client; a program which can be downloaded and used on computers. 'If my current employer discontinues Thunderbird ....Are you saying that the computer is not yours, it is in a place of work and your employer wants to uninstall the Thunderbird program?

Or is this computer yours, at home with Thunderbird installed. Thunderbird has a mail account - work email address - which accesses the work server and offers access to work emails.

re :Can I transfer my current folders and files to a new Thunderbird email that is not related to my work email?

Thunderbird is just an email client. You can add as many mail accounts (email addresses) as you like. If you currently only have one imap mail account (work related) accessed by Thunderbird then add another. 'Menu icon' > 'New Messages' > 'Existing mail account'

You can get free email addresses with eg: gmail or if you have your own ISP (home use broadband) then ISP would have given you a personal use email address.