I'm not creative enough for this, and you can't help me.

Thunderbird (3.0.1) update marks messages unread!

January 26, 2010 in Blog

This is a confirmed bug: https://bugzilla.mozilla.org/show_bug.cgi?id=540554

To fix it, you just need to go in the config editor and set

mail.server.default.use_condstore

to false, supposedly.