Please offer a bit more info on the setup.

What OS are you using on each computer? What version of Thunderbird are you using?

Have you set up the account as IMAP ? That would mean each computer would display whatever is on server.

Have you installed same version of Thunderbird on both computers? Did you set up the mail account as IMAP on both computers? If yes, have you subscribed to see the same folders on each computer?

Do you have Thunderbird profile stored in the default location or elsewhere ?

@Toad-Hall thank you for your reply. I seem to have found the problem, by chance. When comparing Thunderbird Advanced account settings just now, I noticed the PC had the "Show only subscribed folders" checkbox ticked, but the laptop did not. I don't know how that occurred. Closing Thunderbird and re-opening it on the PC (after un-checking that checkbox) has started the process of adding all of the missing folders and emails.

I typed the following before I found the problem. My Lenovo ThinkPad laptop runs Windows 10, and Thunderbird version 128.9.1esr (32-bit - not sure why as it is a 64 bit system), and it shows all of my Thunderbird folders and emails just fine.

My desktop PC runs Windows 10, and Thunderbird version 128.9.1esr (64-bit), and it only shows some of the folders and their emails, with others in italics (empty) and does not show others at all that my laptop shows.

Both laptop and PC are showing Server Type: IMAP Mail Server.

I appreciate your help anyway. Noel