Backup for me from http://getsatisfaction.com/mozilla_messaging/topics/make_thunderbird_3_look_act_like_thunderbird_2:
Here are some tips for making TB3 look & act like TB2:
(1) Switch to "All Folders" view. Use the little left and right arrows in the header bar at the top of the folder tree pane. Or go to Help --> Migration Assistant and then select 'use "all folders" mode'.
(2) Switch to the old, familiar toolbar. Go to Help --> Migration Assistant and then select "use original toolbar".
(3) Turn off indexing. Go to Tools --> Options --> Advanced --> General (tab) and uncheck the "Enable Global Search and Indexer" box.
(4) Open messages in new window (not a tab). Go to Tools --> Options --> Advanced --> Reading & Display (tab). Change the "Open messages in:" selection to "A new message window".
(5) Hide tabs when there is only 1. Go to Tools --> Options --> Advanced --> General (tab) and click the "Config Editor..." button. If a cautionary dialog box pops up then click the "I'll be careful, I promise!" button. Start typing in the filter line of the small window that opens. Type "mail.tabs.autoHide". You will see a seeting with this preference name in the list below. Double click it to change it to "true".
Doing both (4) and (5) will (mostly) turn off tabs.