...
Courier email box (if in standard location): C:\Program Files\Courier Email\Mailbox\your-mailbox.box .
Thunderbird email box: C:\Documents and Settings\....(1)\Application Data\Thunderbird\.....
...
After the migration find a folder called Courex in C:\Program Files\Courier Email\Mailbox\ which contains the address book, mailbox settings (all settings inclusive passwords!) and log files. If the Courier mailbox is in a customized location the Courex folder will be under C:\Courex
After the migration find a folder called Courex in C:\Documents and Settings\....(1)\Application Data\Thunderbird\Profiles\qvu1n3nv.default(2)\Mail\Local Folders\ . In this folder all main and sub-folders, emails and attachments are stored.
...