Same problem for me after upgrade to Gutsy (i.e. Thunderbird 2.0 from 1.5). I deleted training.dat from .mozilla-thunderbird/profile_name folder but it did not solve the problem -- it still freezes when new mail arrives. I would not like to delete .mozilla-thunderbird folder since I need my stored mail.
Same problem for me after upgrade to Gutsy (i.e. Thunderbird 2.0 from 1.5). I deleted training.dat from .mozilla- thunderbird/ profile_ name folder but it did not solve the problem -- it still freezes when new mail arrives. I would not like to delete .mozilla- thunderbird folder since I need my stored mail.