I've been using Thunderbird successfully (since version 0.8, I think) and am now running version 1.0 at home. My PC is running Windows XP Pro. I'm set up for IMAP, and have about 30 IMAP folders on my mail server. There are about 300 messages in the Inbox.
At work I run Thunderbird 0.9 on Windows 2000 Pro.
Two days ago Thunderbird at home became very slow when I opened it and it checked for mail. It took a long time to download the headers. I deleted mail, emptied the trash. When I opened Thunderbird again it was again very slow, and the messages came back, and in some cases were duplicated.
At work, I had no problem, and was able to clean up the duplicated messages.
After reading the FAQ and Knowledgebase, I deleted all the MSF files on the home PC. Thunderbird found the folders and downloaded the headers, but again very slowly (several hours for the Inbox). I've tried recreating the account and even a new profile but the behavior is the same.