The "one size fits all" answer is to move all the messages you want to keep into Local Folders in Thunderbird. Then you know they are stored on your own disk drive and are yours to keep. If the old account uses POP then you don't ''need'' to do this as the messages stored in it are already on your disk. Personally I wouldn't want to leave a dead account in Thunderbird as it would try to connect to its server and annoy me with failed log in attempts. If the account uses IMAP then you must retrieve the messages as they will vanish when connection to that server is lost. To find out what account type it is, go to Tools|Account Settings|Server Settings and see what it says under "Server Type".