I am using thunderbird and just noticed a small thing I thought I would mention.
When I am sorting my email by date, newest to oldest, the conversation drop down doesn't sort the mail in the same fashion. It is always oldest to newest and the newest is all the way at the bottom.
From a usability perspective this is probably not a good default because most likely the last email in the conversation is at the bottom and requires the user to scroll. It also means when you first click on the conversation it doesn't show you the newest email in the conversation if it is collapsed. From a functional perspective it isn't really working in conjunction with the sortable headers above.
That is my 2 cents.
I also prefer my e-mail to be sorted newest-to-oldest, with the new e-mail at the top. However, if I set or clear a quick filter the sorting reverts to the default oldest-to-newest. When this happens, the "Date" header doesn't change, still indicating that the newest is on top. I have to click the "Date" header twice to get it to sort correctly.
I also find it annoying that Thunderbird will automatically move the scrollbar all the way to the bottom, forcing me to manually move it back to the top (the ctrl-home shortcut standard does not work).
Please consider us top-first folk who switched to Thunderbird from other applications (like Outlook).