In the Google search link the first result I see is https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker This kb article explains how to enable and use the Spell checker in Firefox and how to install a dictionary in Firefox (if you need one) since only a few of the language locales comes with a dictionary due to licensing reasons or perhaps because you want multiple dictionary languages . The en-US Firefox comes with a dictionary along with maybe a couple others. The "add to dictionary" is a way to manually add a word (ideally one that you know is spelled correctly) to a dictionary for spell checker to use as the dictionary does not come with all of the words for the language. If you right-click the Post a Reply box in this thread and Languages > Add Dictionaries it should open https://addons.mozilla.org/firefox/language-tools/ in a new Firefox tab. On top it gives you the basics of what each are for <blockquote>'''Installing a dictionary add-on will add a new language option to your spell-checker, which checks your spelling as you type in Firefox.''' Language packs change your browser’s interface language, including menu options and settings.</blockquote> You likely have no use for the language packs as they are a way to have one Firefox install but be able to change the language of the User Interface like menus and such between two or more languages. The language packs so not work with the spell checker.

Thanks James. Appreciated.