Good

LanguageTool - Качественная проверка орфографии.

Just installed it.. it works really well or though it needs work for the more advanced words and slang. It would be great if advanced users could submit their data to better the system. I've been a Grammarly user for 2 years and not accepting feedback is what's keeping it from going from ordinary to superb. So far your Spell-checker is doing a superb job. Will consider going pro after trial period. What's really great is that it supports several languages and this is probably the most interesting point as I write in 4 languages.
Keep up the good work, look forwards to seeing future version.
Why is your app not active in this window?
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Regards,

Installed LanguageTool for Libre Office Writer, but find LT even more useful when filling out web site message fields, like this one, in Firefox. In Writer, the squiggly red underline is enhanced, and grammatical errors are flagged by LT with a different color squiggly underline. In most text entry windows in Firefox, the underlines are solid, and clicking on the word or phrase brings up alternative spellings or edits, or occasionally a suggestion of fixing things.

With squiggly underlines, right-click on the word as usual to get a drop-down menu of corrections. With solid underlines, left-click on it to get spelling, grammar, or style corrections or suggestions. I recommended LT for anyone who wants more than built-in spell checking.

awsome app

The plugin checks for tens of seconds to a minute, and sometimes does not work at all. Perhaps this is because the location is Russia with fenced Cheburnet, but it became impossible to use the plugin.

the add-on slows down the browser way too much. There's an unacceptable long delay between typing letters and the apperance in a text box.

Refuses to work no matter what I do. I have tried Restart Firefox, delete and reinstall the extension, Delete the extension then uninstall Firefox then reinstall Firefox then the extension. Nothing worked.