I think you meant to ask about Thunderbird -- the stand-alone email program -- rather than Firefox -- the web browser. However, please clarify.

Note: When you use Windows 10's restore/refresh feature, you either can leave your data in place or have Windows make a temporary backup here:

C:\Windows.old

If you have a C:\Windows.old folder, your mail data should be in there. For reference: http://windows.microsoft.com/en-us/windows/show-hidden-files

The cleanest way for Thunderbird to get your mail is from the server. If you have been using an IMAP connection, your mail should still be there. However, if you have been using a POP connection, your local folders are the best source.

Even if you need to move local mail from WLM to Thunderbird, the easiest approach is to re-setup WLM and use it to upload your missing mail to an account using an IMAP connection. This may not be the fastest method, but it avoid the glitches and corruption involved with mail converters.

I have to pause here.

I agree with your suggestion: " re-setup WLM and use it to upload your missing mail to an account using an IMAP connection. " ! But my WLM-program is gone and I can't find any WLM download site to re-install it. Microsoft stopped that, I think. PS 1.: I have a C:\windows.old and C:\windows.old(1) folder on my PC. PS.2: Do you have a suggestion on how to re-set-up the WLM-program ?

Hmm, can you find an old Download folder (or old backup) with the Windows Live Mail/Windows Live Essentials installer in it? Random websites claim to have it but it's hard to know who to trust for the file other than Microsoft itself.

If that isn't an option, maybe this Thunderbird add-on will help you get WLM's individual .eml files into a Thunderbird folder: https://addons.mozilla.org/thunderbird/addon/importexporttools/