Update August 2021: Alexander’s add-on isn’t available anymore, a new one can be found at Grammar and Spell Checker — LanguageTool :: Add-ons for Thunderbird
Link: https://addons.mozilla.org/en-US/thunderbird/addon/grammar-checker/
This is a maintenance release that has fixed the compatibility issues with the LanguageTool API v2 (the current one) and the new Thunderbird API. No functionality has been added, but now the add-on actually works.
Development of the add-on have been moved to Github: GitHub - nuald/thb-gramchecker: Thunderbird add-on for checking grammar using LanguageTool. Replaced by https://addons.thunderbird.net/en-US/thunderbird/addon/grammar-and-spell-checker/
Unfortunately, I can’t actively maintain the add-on, but all pull requests are welcomed.
P. S. Dear site administrators, please provide me the wiki access, so I can change the add-on status and link on the corresponding page.