Style and Grammar Checking for Firefox

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Mozilla Firefox supports spell checking, but even for native speakers it's easy to mix up words ("Children resemble there parents") or to make typos that result in grammar errors ("I talks to him yesterday"). LanguageTool is an Open Source style and grammar checker that detects many of the problems a common spell checker will miss. It's available as an extension for Firefox so you can conveniently check text before you submit it on the web.

It's free, no registration is required, and we don't collect your text data. It supports English, Spanish, French, Russian, German, Polish, and more.

After installation, an icon will show up next to your search bar:

To check a text, click on the extension icon while your cursor is in the text field you'd like to check. A pop up will show the potential errors:

Most errors that LanguageTool can detect come with one or more suggestions to fix the error. Click it to have the error replaced in your text.

Known Issues

  • It is not possible to check texts on websites which were loaded before the installation of the extension.
  • A small number of sites like Google Docs is not yet supported - but Google Docs has its own add-on
  • If your text contains an error but the add-on doesn't detect it, that's probably because we don't have an error detection rule for that error yet. Please consider contributing.


The source code of this add-on is available at Github. It's released under the LGPL 2.1 or later.


For Chrome users we offer LanguageTool for Chrome, which works the same.