I just downloaded Thunderbird 3.0 on my Macbook Pro running Leopard, and installed it in
/Applications. When I attempt to start it, the icon opens on the Dock as normal, but I get the following dialog.
I don't have a
~/Library/Application Support/Thunderbird directory at all, let alone a .parentlock file. While I didn't expect it to help, I did reboot my system. And sign out and back in. And close all programs besides Thunderbird.
Earlier versions of Thunderbird have worked just fine.