There is no such thing as Thunderbird webmail so no idea what you are talking about there. Email providers have webmail access to their email service but Thunderbird is client software not a provider. Have you subscribed to these webmail folders in Thunderbird? File-Subscribe

Thanks for your response. I am using Fastmail as my webmail (?) and Thunderbird as my client email (?) . I don't know what you mean by subscribe. I don't see how to subscribe the webmail folders in Thunderbird. I cannot find "File-Subscribe" on my Thunderbird client software. Thanks for any help you can give me.

Subscribe is under the File menu. First off do you have the account setup in Thunderbird as IMAP or POP?

Airmail, Thanks again for the response. Thunderbird (TB) is set up as IMAP. I found subscribe on TB but two of the folders currently on the webmail (fastmail) are not visible on the TB subscribe list. Do I need to set new folders up on TB on my pc to then be replicated on the webmail rather than set up new folders on the webmail to be replicated on to TB?

You should not have to. Normal IMAP operation is you are just viewing the folders and messages on the server remotely with your email client on your computer. I guess it could have something to do with where in the folder structure you created these folders on the server.

I now recognize that I cannot add any subfolders to TB on my laptop pc. I am guessing that this is the same issue keeping Fastmail from setting up new folders on TB. I have set up and used subfolders in TB until my recent move to a new laptop pc and a new version of TB.