Some Linux distributions like Ubuntu install a lot of dictionaries and Firefox finds them and makes them available.

You can check these locations for globally installed dictionaries:

• /usr/share/hunspell/
• /usr/share/myspell/

See also the DICPATH environment variable.

Note that there is a difference between language packs that provide translations for strings in the user interface and dictionaries for spell checking. They also have their own tab on the about:addons page.

• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker
• https://support.mozilla.org/en-US/kb/use-firefox-another-language

There is also a third setting for the preferred languages for web pages.

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