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"There was an error downloading New Mail Attention"

And since there was an error DOWNLOADING it, why are you leaving a star to my extension? This is an issue with Mozilla, not with New Mail Attention.
Anyway, you can download the.xpi file from addons.mozilla.org (https://addons.cdn.mozilla.net/user-media/addons/191033/new_mail_attention-1.2.1-tb+sm.xpi?filehash=sha256%3Afc7a3098ee2159a77d87956ed13bf8db71b3995f3d906063862cf047f2d1f6c1) and install it in Thunderbird selecting "Install Add-On from file".