I like the current folder views "Unified Folders", "All Folders", "Favorite Folders", etc. But it would be preferable if we could have a view where we can organize all of it ourselves. I personally have a lot of emails connected to thunderbird, and it works really well, but it leads to even the unified folder view looking really, really ugly. As I have a full list of all of my emails there, it feels very disjointed and clunky.
It would be nice if I could make my own groupings and folders for structuring my folder pane, similar to how we already can in so many other parts of the interface.
I want thunderbird to be my one-stop shop for all things email and chat, and it currently is, I currently love the look of the client, it was the one thing holding me back from using it before. But I really think the client could go one step beyond and allow for more grouping, sorting, organizing, and customizing of that left folder pane. This same problem persists with the calendars too, too many calendars leads to a really clunky and mess list of calendars.
So maybe we could get some sort of custom view system, or the favorites system could be revamped.