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: https://github.com/nuald/thb-gramchecker
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.