In Settings/General, Incoming Mails, App Icon Options, select 'count of unread messages'. That seems to display the number in the icon that's useful.

That value is checked, but my taskbar icon doesn't display any numbers or any indication of new emails. There is little bar underneath it that highlights when thunderbird is not minimized, but that's it. I wonder if its some kind of windows setting that is hiding the values. I went through the windows settings for taskbar behavior to see if I could figure anything out but still now showing anything.

Check Windows Settings for System/Notifications and Personalization/Taskbar behaviours, Show badges.

Yes one of those windows must have done the trick, I was playing around with them not sure which one made the difference but its working now thanks !