Firefox by default ships with a spell checking dictionary that matches the language version of the browser. If you download the Firefox US release you end up with the US-English dictionary for instance. This usually works out of the box but may be buggy at times. You can check Fix Firefox Spell Check Not Working to correct possible spell checking issues.
Some users may want to have multiple dictionaries at their disposal, for example if they have to write or spell proof US-English and British-English articles on the web, or if they communicate in at least two languages.
How is that done?… “