A language pack is a option for to have one Firefox install but be able to change the language of the User Interface or UI of the menus or such. This one is for Canadian English. Some people install a language pack by mistake when they need to install a dictionary https://addons.mozilla.org/firefox/language-tools/ for the inline spell checker to have a dictionary to use for example as only a few locales like en-US comes with a dictionary due to licensing reasons.