When you used IMAP the messages are still on the server of your old mail provider.

I know, but since I'm not a customer there anymore, they have deleted my account and all my mails (I'm sure there is a backup, but they just say it's gone). So I was hoping Thunderbird kept a "hidden" backup somewhere...

Thunderbird might still have them as deleted. As long as the folder has not been compacted.

So open your profile folder (help menu > troubleshooting information and click the show profile button. Ensure Thunderbird is not running.

open the imap mail folder in the profile. Then open the appropriate imap mail account folder.

Locate the file inbox (I assume it is the inbox folder). There will also be an inbox.msf file. that is not relevant but might help locate it.

Copy the file to another location, like your desktop. open in it a plain text editor. I suggest you download and use notepad++, but windows default notepad might suffice, depending on file size but a wordprocessor like word or libreoffice will just make a bad situation a total disaster.

use the menu commands for find and replace to find all occurrences of;

X-Mozilla-Status: 0008

with

X-Mozilla-Status: 0001

Then install the import export tools add-on https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools/

Use the add-on to import any mail the file contains from an mbox file type.