I was compelled to upgrade Thunderbird to 3.0 today. Maybe my desktop has a thing for odd numbers. Windows 7, Firefox 3.55, Office 2007.
So far I can say that my settings and local mail are all intact. The layout of things is a bit different. It does the “smart folder” thing that has been part of Outlook and Mail.app for a while. This can be nice as I have three separate accounts set up in Tbird. Smart folders automatically allow me to display all three inboxes merged together.
My Lightning calendar plugin had to be disabled but I wasn’t using that anymore anyway.
Time will tell – we’ll see what else I find!