Tomorrow’s nightly build of Thunderbird 3.0 b3pre adds support for the IMAP Compress extension – see Bug 401673 and the rfc for more info. If the server advertises COMPRESS=DEFLATE, we will do compression of incoming and outgoing data. For lower bandwidth connections, this should be a nice win, especially when we select large folders and fetch flags. I’ve seen reports of > 5x compression for the flags response. To turn it off, you can use the config editor to toggle mail.server.default.use_compress_deflate to false.
The latest versions of the Cyrus IMAP server supports COMPRESS=DEFLATE. Fastmail.fm has rolled out this support already.
Many thanks to Bron Gondwana who developed the patch very quickly and cleanly, despite having no experience with the Mozilla codebase.