Das Language Tool funktioniert recht zuverlässig in Hinblick auf die Orthografie, die Grammatik-Hilfe ergibt bisweilen recht abenteuerliche Vorschläge. Ich rate all jenen Mitbürgern dringend zur Installation, die bar jeglicher Bildung aggressiven Unfug, Beleidigungen und extremistische Hetze in die sozialen Netzwerke posaunen.

Early days for me but it seems to do a reasonable job.

This is not a local spellchecker (all browsers use to have local spellcheckers)
"LanguageTool" forwards all of your text to third party servers (wasting data and adding latency) and does not show you errors until responses are received from those servers (huge lag)
Mobile Firefox removed its spellchecker because Android had a spellchecker, but Google removed Android's spellchecker because most browsers has spellcheckers.
LanguageTool is now the sole result for https://addons.mozilla.org/en-US/android/search/?q=spell%20checker but our business does not allow apps that forward text to remote servers.
Now 4 stars due to responses about local servers

the Wonderful tool with excellent experience