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por Alexander Ihrig el Nov. 28, 2019 · enlace permanente

You should not rely on Lightnings own locale strings. This leads to XUL errors for many users after upgrading to Thunderbird 68.*, when Ligntning is (maybe temporarily) disabled.

Have a look in this topic:
https://thunderbird.topicbox.com/groups/addons/T5b15e51d21e1a3e8/is-there-a-manifest-key-for-require

And the XUL error by your add-on:
https://www.thunderbird-mail.de/attachment/10900-thunderbirdtoolbarbutton-jpg/