The firefox-locale-en package is Recommended by language-pack-en-base, which is included in Lubuntu.
You can safely remove it if you wish. Be aware that the package manager may (helpfully) reinstall it for you in the future.
$ apt-cache rdepends firefox-locale-en
$ apt-cache show language-pack-en-base
Description-en: translations for language English
Translation data for all supported packages for:
This package provides the bulk of translation data and is updated
only seldom. language-pack-en provides frequent
translation updates, so you should install this as well.
Task: ubuntu-live, ubuntu-usb-live, kubuntu-live, kubuntu-active-live, kubuntu-active-live, edubuntu-live, edubuntu-dvd-live, edubuntu-usb-live, xubuntu-live, lubuntu-live, ubuntustudio-dvd-live