The older version of Thunderbird had a nice option to mark as unread which you could easily click on. Now you have to go into the "mark" menu and click on mark as unread. There's the letter M which would work, BUT if you are a right handed user using a mouse, that doesn't work because you have to keep moving your right hand off of the mouse to hit M. Every time I used Thunderbird this ticks me off.
I'm requesting that you add 2 options to view->toolbars->customize "mark as read" and "mark as unread". Also, it would be good if you picked a shortcut letter, instead of M, that could easily be hit with your LEFT hand while keeping your right hand on the mouse. Something on the left hand home row would be very good.