First thing is Unread messages should be shown first, having to manually select unread all the time is frustrating, especially when Thunderbird buries them inder messages you've marked read already through the google mail mobile app, second they should be more arranged by time/date received I see messages from several days ago being shown before messages that I just minutes prior on a daily basis. More connectivity with other clients to arrange folders the way you have them in the other Clients, Google, outlook, and several other Clients do this. Also an option in settings that when you click close for Thunderbird, sends it to the tray instead of fully closing, I know I am not the only person that instinctivey closes windows when done not thinking about the fact right away it might be something you want to run in the background, because most of the time to save RAM you close apps out and not just minimize them.
