In Settings/General, Incoming Mails, Customize.. there is a checkbox for Message Preview Text. Is that checked, and does it work?

I could not find "Incoming Mails". The nearest settings pane is the one in Chat. As per screen shot. If this is the setting you meant, it already is set at "with sender's name and message preview"

See the attached pictures. This is for W11, but it might be different on OS X if notifications are controlled in OS X System settings.

Thank you for your reply. I was mistaken. Incoming Mails exists. However, the option for notifications is from the main system settings.

Nevertheless, In the System Settings in Notifications in MacOS, I've already got the option "Show Previews". However, the issue persists.

I also came across this post from four years ago and the user also suffers from the same issue: https://support.mozilla.org/en-US/questions/1297835

Can someone from Mozilla/Thunderbird Team resolve this issue for MacOS users?

That forum topic is fairly comprehensive, and the bugzilla link indicates there's been no resolution in the last two years. If cannot be resolved in OS X settings, perhaps there is a standalone notification app (there are many for Windows) that would offer those options.

Thank you again @sfhowes for your reply.

you mean like an add-on? I did search on https://addons.thunderbird.net and couldn't find anything.

do you have the bugzilla link?

See https://support.mozilla.org/en-US/questions/1297835#answer-1339288

I'm not really familiar with OS X apps. There are plenty of full mail clients, and a few notifier apps that are gmail-only, but no current ones I found that work like the Windows POP Peeper.