My complaint is it should store each email in its own file (like NNTP readers do). This makes it much more amenable to backups, search, and makes it a lot less susceptible to corruption.
Instead, Tbird stores one folder in one gigantic file. One threading error, and poof! It's gone. Happened to me several times. Yes, I filed bug reports. No, it was never fixed (at least the Tbird developers denied this was a problem).
Another minor thing I'd like to see is don't hide everything in multiple places.
There's still no way to backup my account settings.
Instead, Tbird stores one folder in one gigantic file. One threading error, and poof! It's gone. Happened to me several times. Yes, I filed bug reports. No, it was never fixed (at least the Tbird developers denied this was a problem).
Another minor thing I'd like to see is don't hide everything in multiple places.
There's still no way to backup my account settings.