You can't remove the default language of your installed locale, so if you use the en-US Firefox version then you can't remove this language. You would have to install the en-GB Firefox version as I see this language as a choice in the screenshot to get rid of the en-US choice. You can possibly install the en-US dictionary if you need this for spell checking. The en-GB Firefox version doesn't come with a dictionary, so you need to install this dictionary like you probably already have done. Note that language packs only provide translations for strings used in the user interface and have nothing to do with a dictionary. You can look here for dictionaries: *https://addons.mozilla.org/firefox/language-tools/ ---- *Options/Preferences -> General -> Language and Appearance *https://support.mozilla.org/en-US/kb/use-firefox-another-language *https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages