The best solution is eSoftTools IMAP Backup & Migration Software that helps to import/add Proton Mail to Thunderbird. The interface of this software is designed to be user-friendly. Moreover, it also creates a new folder on the IMAP server to save important IMAP mails. It also supports IMAP backup to Outlook without any error. Moreover, you also get a free demo facility in which you can save up to 25 items from each folder without any charge.
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https://www.esofttools.com/blog/import-add-proton-mail-to-thunderbird/