I also use MozBackUp to do this. It works great, and conveniently will export all your data (emails, configuration settings, accounts, etc.) to one file that you can then copy to the new machine, install MozBackUp there, and then import (or restore) from that file.

MozBackUp can be found here: http://mozbackup.jasnapaka.com/

Hope that helps!

I have just completed the change to new computer and used MozBackUp
It was the easiest transfer i have had to do.

i have to HD's in my box, both were once masters in their own boxes. i want to get the address book from the slave and use it only the current master.

Hmn, I'm trying to use MozBackup to do this but it will not import from the old HD... the IMPORT button is greyed out.

I suspect I have to try to reinstall that old HD and export from Thunderbird, then import the files-- is that right? Alas!

just copy old file called inbox and sent in your old directory and dump it in the new install directory.

set windows preference to see hidden files and folders as THUNDIRBIRD mail folders should be in C://document and settings/users/app data/your username/mozilla/thundirbird/default"yusbhx"
the appdata folder is a hidden folder so yu need to have windows set to "show hidden files and folders.
copy inbox
go to your new directory (you can find new directory location in account setting option) still need to have windows set as show hidden files and folder
paste inbox or sent file in location
voila

I would support this question. There may be bits and pieces of information around the T'bird web pages that can be assembled into a partial answer with some study and a bit of difficulty. But there is no clear explanation of the process.
The process is not made easier by the way T'bird effectively hides the actual location of email folders. The location is not clear in the options settings.

I keep all my own data on a separate partition D: and I thought I had directed all the email folders to D: as well. But no. When I migrated to Win 7 recently, I forgot (if I had ever fully realised) that T'bird had buried my inbox and address books somewhere in the Microsoft mess of folders hung off My Documents.

The near loss of all my address books was the main disaster. It took me some hours to recover them piece by piece from the Windows.Old folder created in the Win 7 upgrade.

Some years ago I had heard of MozBackup but I had forgotten all about it. Why should I have to remember such tricks when installing a new OS? I have quite enough else to think about without wishing to worry about T'bird's data placement idiosyncracies. If MozBackup is crucial then let it be rolled up into T'bird.

I aim to manage my data SEPARATELY from the OS on a SEPARATE partition which I back up regularly. I learnt the importance of that many decades ago when mainframe disk packs were just 4 MB.
So when reinstalling the OS on C: , the last thing I want to have to worry about is bits of crucial business data buried somewhere in C: by Thunderbird. Address books are CRUCIAL business data. Let me keep them on D: and back them up with all my other data.
If I lose partition C: I expect to reinstall Windows.

Protecting one's own data, emails, address books etc would be much easier if T'bird gave the user proper and full control over where they are placed.