KeePasXC-Browser doesn't work correctly for me in Thunderbird, so I decided to use the recently created keepassxc-mail add-on. This currently requires some manual interaction:

• Copy the file kpnm_firefox.json in %LocalAppData%\KeePassNatMsg to kpnm_thunderbird.json and change the name and allowed_extensions value (sample file)
• Add a new registry key de.kkapsner.keepassxc_mail under HKEY_CURRENT_USER\SOFTWARE\Mozilla\NativeMessagingHosts and point the (Default) value to the previously created file

Could you please add support for these steps in the KeePassNatMsg options dialog so that happens automatically?

Also, the add-ons author noticed a difference in behavior between how KeePassXC and KeePassNatMsg handle native messages: kkapsner/keepassxc-mail#11 (comment) This may be expected behavior, but it also could be a bug in KeePassNatMsg.