It’s available at Grammar and Spell Checker - LanguageTool – Get this Extension for 🦊 Firefox (en-US) - if you have installed the old Firefox add-on, you should get an update automatically rather soon.
Unlike the previous versions, this one can underline and fix errors directly in the text:
If you prefer the old version, it’s still available at Grammatik- und Rechtschreibprüfung - LanguageTool Versionsgeschichte – 25 Versionen – Add-ons für Firefox (de) (1.0.46 is the latest version before the rewrite).
If there are any issues, or if the update doesn’t work, please let me know.