According to
Thunderbird : FAQs : Migrate from Mozilla or Netscape
migration can occur only once... the very first time you run Thunderbird, when the first profile is created.
Once you have created your Thunderbird accounts, you have two choices. The first one:
If you "missed your chance" to import your profile via the Import Wizard and you would like to do so later, you can bring up the Import Wizard again by exiting Thunderbird, using the Profile Manager to delete all of the profiles listed, exiting the Profile Manager, and then launching Thunderbird again. This method is recommended only if you have been using Thunderbird for a very short time and you don't mind losing your account settings (if you've already set up one or more acccounts) and access to any messages you've already downloaded.
The second option would be to manually copy over the Mozilla profile's MAIL\"mailaccount" files (Inbox, Sent, Drafts.... etc.) from the Mozilla profile to the Thunderbird profile's MAIL\"mailaccount" folder.
For example, in WinXP, that could be a copy of the files Inbox, Sent, Drafts, and Templates, from
[tt]C:\Documents and Settings\Alice\Application Data\Mozilla\Profiles\default\xxxxxxxx.slt\Mail\pop.MyIsp.net[/tt]
to:
[tt]C:\Documents and Settings\Alice\Application Data\Thunderbird\Profiles\default.xxx\Mail\pop.MyIsp.net[/tt]
In Win2000 and WinXP the Application Data folder is hidden so you will need to go to your Windows Folder Options (accessible through Control Panel or from any Windows folder's Tools menu) > View tab and,
under Hidden files and folders, select "Show hidden files and folders". Also UNcheck the box, "Hide File Extensions for Known Files." so that all file extensions are shown.
Just copy the files without extension, for example, copy the file
Inbox, not Inbox.MSF! and
Sent, NOT sent.MSF. When asked to overwrite the existing files say YES if you have no mail in the Thunderbird profile you wish to save.
If you already have mail in Thunderbird, rename the Mozilla files OLDinbox, OLDsent, etc before copying them over to the Thunderbird profile.
You can use the information here as a general guide:
http://home.att.net/~cherokee67/ns7mailtrnsfr.html
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