Mozilla Thunderbird (email client) has an add-on FileLink that allows large attachments to be stored in various user cloud accounts.  I tried setting this up for Box and got the following error:

There seems to be a problem with this app.

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Error: invalid_client
Show Error Details

    scope=root_readwrite
    folder_id=
    response_type=code
    redirect_uri=https://3f09a9a073315ab3f3a9e21bcb23cef791ba9c9b.extensions.allizom.org/
    client_id=9ce6u2ggoq8s9l2jil4dvifp2cpswin7

I know Box is not responsible for problems in 3rd party software, but please explain what "invalid_client" implies.  I assume this client code lives at https://3f09a9a073315ab3f3a9e21bcb23cef791ba9c9b.extensions.allizom.org and is trying to communicate with Box.  I provided the Thunderbird add-on with my Box id and password (Perhaps a mistake, but I trust Mozilla.) and the validation of the account seemed to work so I assume this error occurred after signin.