It appears that there is some sort of interoperability issue between Thunderbird and the IMAP server. When downloading rather large IMAP folders, thunderbird takes a very long time to display list contents. I get "opening folder, sending credentials" then it switches to "Compacting Folder" and hangs for several hours before displaying the contents.
This is 1.07 under debian; I *do not* have this issue with evolution on the same machine.
FYI, we're probably talking somewhere around 5-10,000 messages in these folders.