Shortcut for Thunderbird's unread mails
Solution 1:
As far as keyboard shortcuts are concerned I don't think there is what you want, but a mouse click on the title unread messages is just as fast.
And here you have all the keyboard shortcuts available and there are many. https://support.mozilla.org/en-US/kb/keyboard-shortcuts#w_moving-around-thunderbird
I forgot one thing, maybe you don't have the filter bar active ? it is in french but it's very probably at the same place
have a nice day !