Rated 4 out of 5 stars
Helps avoiding missing unnoticed unread emails. As long as there is no toggle shortcut and a toggle button to toggle quickly the add-on's "Unread (Advanced)"-command, this add-on is best used together with the "Folder Pane View Switcher" add-on. You get to the "Unread (Advanced)" view with just one click on the back arrow. ;-)
BTW, in my FF11 none of the options (either clicked or unclicked) stick after they are okayed.
I still miss an add-on that keeps indicating DIRECTLY on the TB interface if there is still some unread email in any folder (I use eight different accounts, and those that are collapsed don't betray at all if there is still an unread message in them).
Maybe some blinking sign or a blinking number (displaying the number of unread mails) in the status bar would help to draw attention to it that there are still unread emails somewhere. The number would stop blinking when there is no unread mail left.