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

Hey I'm learning but it dos not work here

I do a LOT of writing, a large amount of it as short essays, and commenting on YouTube and social media platforms. It is very important to me to compose texts using the best language grammar, spelling, punctuation, etc. I primarily write English text, but also German, Spanish, and French respectively. I also use a lot of Latin phrases, but being very new to this extension/app, I don't know if it is supported. I have read many reviews about this extension. So far, I've been using it on all my compositions, to the limit allowed in the free version. I just need to clean up, and check for errors, as I appreciate and consider any suggestions it makes. So far I love it. Any flaws I might have seen are quite petty, and I easily work around them. I consider myself able to command English, and language in general with well above average power. Nevertheless, this extension is already helping me improve. I give for stars, to allow room for well deserved improvements, which I KNOW are coming. Then I will upgrade to Premium. This is a fine App. IMO.