[Dovecot] Thread sorting problem

I've just installed Dovecot on a Debian Sarge machine and am seeing some very strange thread sorting.

In Mozilla Mail and Thunderbird on both Fedora and Debian clients my threads served up by Dovecot look like this (number denotes message time):

|-11 | -12 |-2 |-3 |-4 | -5 | -6 | -7 | -8 |-1 -9 `-10

Where the exact same threads served by Sendmail/imap look like this:

|-1 |-2 |-3 |-4 | -5 | -6 | -7 | -8 |-9 | -10 |-11 | -12

Squirrelmail shows the threads correctly in both cases too, so I suspect there might be something funny going on with how Thunderbird handles Maildir threads. I notice that Dovecot doesn't support the THREAD=ORDEREDSUBJECT capability, might that have something to do with it?

thanks, Greg