Changes

# cpu usage - is it high (pegged), zero, low, or other?

# run netstat -a -b -o | grep -i thunderbird (or for windows, netstat -a -b -o in a dos command window) - are connections shown from thunderbird to ldap, imap mail server, or other protocols?

# If the problem is easily reproduced and you have extensions installed, does the hang happen with Thunderbird started in [http://kb.mozillazine.org/Safe_mode safe mode]? If it can't be reproduced in safe mode then the problem is probably an extension ... but extensions normally aren't the issue.

# is gloda indexing enabled or disabled? (Tools|Options|Advanced|General)

# are your accounts pop, imap, or a combination?