The installation is slightly more complicated than the usual add-on installation. The reason for this is Language Tool needs to run as a local server. There is an option to use a remote server but this would mean that the email contents would be transferred back and forth to that server which obviously raises privacy concerns.

The developer of the grammar checking add-on for Thunderbird has published explanations on how to setup the local server and configure the add-on in Thunderbird to use that server. In short: All it takes is to run the server from the web start option provided at the Language Tool website. This will download the files to the local computer system and provide an option to start the tool as a server (File->Option and select “Run as server”:) The server settings need to be changed in the extension’s preferences.

Grammar Checker is an experimental add-on that can be downloaded from the Mozilla website.