I have a confession to make: I've stayed away from using Thunderbird for my desktop email client despite my apparent Mozilla advocacy. The reason: I couldn't for the life of me find a "purge" button to expunge emails marked as deleted for my IMAP accounts.
I still use Outlook Express and even recently tried out Outlook 2003 to see what the fuss was all about. Well, yeah Outlook 2003 looks pretty in a WinXP-ish sort of way, and the 3-pane layout really does give you bigger logical screen real estate to manage and read emails. Too bad I still can't figure out why it doesn't work with my university and primary IMAP email accounts. It works for one other IMAP account, which leaves me really puzzled because the account settings were all imported from Outlook Express. I even tried following special instructions for accessing those accounts on Outlook, to no avail.
Anyway, back to Thunderbird. One thing I always missed in Thunderbird ever since those days when it was still called Minotaur is the "Purge" button in Outlook Express, which expunges emails marked as deleted. I wasn't the only one, apparently, from this thread at the MozillaZine forums. From a usability point of view, I find a 1-click access to a "purge" button highly convenient, and "Compact this folder" doesn't exactly convey the impression that something is going to be deleted (or more correctly, "expunged" as in the IMAP RFC.
Well, the good news is, someone has already submitted a bug report to add an expunge button to Thunderbird here. Until then, I'd probably be using Outlook Express on Windows, or until I get some insidious Outlook Express worm or virus, whichever happens first.