You have two choices.

IMAP or POP to receive email.

IMAP is synchronized to the server so you can each see exactly what you have done, including reading of emails. POP is not synchronized so you will have no idea what the other party has done, or what replies they have sent.

Thanks Matt, We just switched to IMAP, does that mean seeing the messages unread on my specific station is no longer an option? Thanks, Cindy

Only if you read it first! IMAP is server based. You view messages remotely on your computer or devices. Anything you do on the server or from any device is reflected everywhere. So if you read a message from anywhere it is marked as read everywhere.

CindyRM said

Thanks Matt, We just switched to IMAP, does that mean seeing the messages unread on my specific station is no longer an option? Thanks, Cindy

Correct :)

Is there a way to use POP instead of IMAP if we use gmail?

One method: Put a copy of all incoming emails into 'Local Folders' mail account Inbox. Leave the ones in imap account alone. Make all emails in Local Folders Inbox as 'unread'.

Then you can open those emails in 'Local Folders' account and move email to suitable folder when read. So Local Folders Inbox only ever contains unread messages.

AS the person in office reads emails, the imap account will update to reflect this, so you know what has been read by office.

Another method: Both of you could stop marking open emails as read. In thunderbird Menu icon > Options' > 'Display' > 'Advanced' uncheck 'Automatically mark messages as read' click on OK

Then choose to use a different methodology of working. example: Determine which person has responsibility of marking emails as read. You could create new folder called 'CindyRead' Put a COPY of all emails you have read into that folder. The person in Office can see what you have read, then if person in office has responsibility, they mark emails as read.

If you use gmail POP, then neither of you will know what the other is doing. All emails are stored in Thunderbird on respective computer. Pop accounts can only access server Inbox to download new mail. You must set up to 'Leave mesasges on server' so each of you can download emails and you would also need to use the 'recent mode' in gmail settings. Any emails currently stored in any other folder on server will not be accessible until they are moved to server Inbox. Sent emails are not stored on server. Info on recent mode: https://support.google.com/mail/answer/7104828?hl=en