Rated 5 out of 5 stars

Excellent!
I run a local server as explained on https://dev.languagetool.org/http-server. This way, Thunderbird, LibreOffice and Firefox use the same server, and LanguageTool works offline.
The proposed corrections in English and French always make sense and go far beyond native correction tools.

This review is for a previous version of the add-on (4.2.12).