Don't know if this is easily corrected but once a dark theme like BlackFox or LavaFox or LavaFox Purple is applied, the menu when clicked is dark with dark text making it unreadable. The new version of X-Notifier with the dark theme also doesn't display the "letter icon" either. It does when the default theme is applied though.
I know this can easily be fixed just by not applying the theme but previous versions remained unaffected with the them.
FWIW - This is my favorite add-on for firefox and simply the best out their for on-line webmail.