SOLVED Newbie:
I finally got fed up with windows and switched to Linux Mint 19.3. So far good. However I had saved my emails in Thunderbird using Lighting. Now under Mint when I try to import the profile back to Linux Thunderbird I get a invalid profile error Any help would be gratefully received.

No idea what Lighting is. Doesn't Thunderbird have a simple export?

That's what I've done, File->Export and put them into a CSV file.

Do you mean the add-on Lightning? I recommend if you can to just perform a simple export.

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I've never even done that - I just pick up the profile and copy it to whichever. Works in both directions - there's how-to's on the site.
If you no longer have access to the Windows filesystem, I can't help.

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Yes. You Should be able to do either of the suggested solutions if you have access to the existing profile.

Caveat: The versions should be the same for the “copy the profile” option.

Lighting is/was an add-on to provide a calendar...its been folded into the base application recently. I don’t believe it has anything to do with exporting emails, tho it may be involved when exporting calendar entries.

If your account was set up as IMAP, you should only need to set up the account on your new ‘puter...the emails are on the server.

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Folks you missed the point; I saved my profiles under win 7. I tried to move them over to Mint. Error was an unrecognised profile. I finally figured out how to do it my deleting msf index files and had Thunderbird rebuild the profile. I have been able to save 80% of my emails.