Firefox 52 is planned to be released tomorrow. The current version of the LT add-on doesn't work properly with it, so I've removed it from languagetool.org for now. The problem is that the settings (language selection, words to be accepted etc.) won't get saved. A fix has already been found, but you cannot just upload it at http://addons.mozilla.org, it needs to go through a code review first, and that can take days or weeks... I'll add the link to the add-on back once the bug fix has been reviewed and a new version of the add-on is online.