I have upgraded to tbp 3 today (09/12/10) and I've noticed that I am losing emails that I archive. I have emails in the archive from the 8th 12, so I have effectively lost two days of emails.
I tested this by emailing myself and archiving the email. It disappears. It isn't in the archive 'folder' nor can it be found by searching. I can get the test email back by ctrl+z, but that only works for the emails archived in this session.
I've also tested this in RC2 and it has the same problem.
I've set tbp3 to never delete emails.
I like the archive idea, (never delete, search, etc)
Archive about:config settings:
Solution is to change: