Hi Antonella,
Welcome to the Box Community, I'm happy to help!
It looks like this integration was developed my a third party and not by Box. Unfortunately, since we don't have control over their software, we have no visibility into the inner workings of how this integration works. I would suggest opening a support ticket with Mozilla in regards to Thunderbird. They should be able to help you out with the issues you are experiencing.
Thanks for reaching out and if you have any questions, please let us know!
Best regards,