Was there an Export option on the Windows 8 version? I don't know of any built-in tools to export and import messages between Thunderbirds.

I would do one of the following:

1. For a static, one-time copy, I'd copy the profile from the Windows 7 machine to the Windows 8 machine. See https://support.mozilla.org/en-US/kb/profiles-tb
2. However, that does it now, today, but you'll forever be struggling to keep the two machines in step. Much better is to use an IMAP-connected account that both machines can see. Both will see the same set of folders and messages and that account will automatically remain in step. Anything you put into it on either of the two machines will shortly be available on the other.

If your current mail provider doesn't offer IMAP, you can get a free account from gmail or gmx, to name but two.

Zenos said

Was there an Export option on the Windows 8 version? I don't know of any built-in tools to export and import messages between Thunderbirds. I would do one of the following:

1. For a static, one-time copy, I'd copy the profile from the Windows 7 machine to the Windows 8 machine. See https://support.mozilla.org/en-US/kb/profiles-tb
2. However, that does it now, today, but you'll forever be struggling to keep the two machines in step. Much better is to use an IMAP-connected account that both machines can see. Both will see the same set of folders and messages and that account will automatically remain in step. Anything you put into it on either of the two machines will shortly be available on the other.

If your current mail provider doesn't offer IMAP, you can get a free account from gmail or gmx, to name but two.

The problem is to make a one time transfer of old, saved e-mail messages from the Windows-8 machine to the Windows -7 machine.

The best way to do this is to copy the old profile to the new machine. But if you have been using the new machine it will already have its own profile, and doubtless messages in it you want to keep, so overwriting the new profile with the old won't be particularly welcome.

Using an IMAP account, if only temporarily, is one possible route, but a bit cumbersome if you have a great many messages to shift.

There is another approach, but it will require some work on your part to find your old profile, identify the relevant mail store files, then copy them to the new machine and import them. This add-on:

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

can help with the importing of your mailbox files.

Info on the profile:

https://support.mozilla.org/en-US/kb/profiles-tb

A less obvious route is to install the old profile on the new machine, so all your old messages are transplanted, then merge in the new profile's messages, again by IMAP or import of mailbox files. The point being to transplant the bigger of the two mailstore sets, and import into it the smaller of the two mailstore sets.