After I did Step 4 in Firefox 56.0.01, I got this message:
LanguageTool TEST - Grammar and Style Checker
Reading manifest: Error processing offline_enabled: An unexpected property was found in the WebExtension manifest.
After going to Begriffsschrift – Wikipedia, selecting the first paragraph of the article and clicking the LT icon, I see the following error message (in German):
I really like the automatic checking. It makes this plugin much more useful because I can instantly see if there are issues without having to remember to press a button.