Typically firefox is installed from the Arch repos. But in the extra repo of Arch there are packages like firefox-i18n-en-gb or firefox-i18n-en-us or firefox-i18n-uk and so on. Please see the attached screenshot given below for more details. Are these required to be installed along with Firefox? When should they be installed? What is their use?
I am using Firefox with en-us language encoding.
I don’t know how it’s in English versions, but at least when I select Finnish as system language, firefox-i18n-fi is always installed. 
Typically they localize the program. Like language, date- and curreny-format.
So if we have the base firefox package installed from Arch repos and we have english as the locale and language is this packages, firefox-i18n-en-gb or firefox-i18n-en-us required?
If we have firefox-i18n-en-gb or firefox-i18n-en-us installed what do we get as extra? If we do not have either of them installed what are we missing out on?
You don’t have to install the localization package, firefox will work in English anyway. It is up to you which localization you wish to use, if any.
