How can I import Thunderbird mail folders into another instance of Thunderbird?

Solution 1:

You just need to copy the full profile between both instances, then select the profile you want to use with:

thunderbird -P [profile]

if you omit "profile" the graphical profile manager will ask you which profile you want to choose.

If you just want to overwrite one profile with the other, just remove the one you don't need.

Chech the following link to locate where are your profiles; you haven't said which OS are you using:

http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Solution 2:

1. Just to be safe, backup your importing (Windows 7) Thunderbird profile - I use MozBackup.
2. In your importing (Windows 7) version of Thunderbird, create an empty new folder with the same name as the folder you are going to import (let's call it In2010).
3. Find the files In2010 (with no extension) and In2010.msf in the Thunderbird profile mail folder you are exporting from (they contain the messages you want to transfer) - in Windows 7 they are in C:\Users\\AppData\Roaming\Thunderbird\profiles\\Mail\pop.yourISP\
4. Copy these two files into the profile mail folder of the importing version of Thunderbird - so you overwrite the two files (In2010 and In2010.msf) you created in step 1. Done.