If not appearing on Thunderbird, it may be that there is an error in the setup. Check the Fastmail settings for email accounts and then post any error messages here. Here are settings I found on the fastmail website; https://www.mailjerry.com/fastmail-imap-settings/

Thank you for replying. I use a laptop. I have used Thunderbird for over 15 years. First time I've had this problem. All emails were there when I closed Thunderbird , and shut down the laptop before going to bed. I'm told messages are being downloaded, but messages don't appear.

I am not familiar with Fastmail, but my recollection is that mail,messagsenging.com has been replaced with new server names. That may be wrong, but I suggest you try the settings from my response as a possibility. Do let me know, I found similar info at

https://www.fastmail.help/hc/en-us/articles/360058753054-How-to-set-up-Thunderbird

I tried the other server name. P/W kept on being rejected. Changed p/w and it still happened. Switching back to mail.messagingengine.com and I could log in. Posts still not showing.