Some websites disable the browser spellchecker. Why? Who knows.

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Grammarly will help you communicate more effectively. As you type, Grammarly flags mistakes and helps you make sure your messages, documents, and social media posts are clear, mistake-free, and impactful.

Note that you need to install a spell check dictionary and not a language pack.

A language pack provides string translations for the user interface.

• https://support.mozilla.org/en-US/kb/use-firefox-another-language

This is also different than choosing the preferred languages for webpages.

• https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

Dictionaries that you install appear under "Add-ons -> Dictionaries".

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.