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.
Code:
$ apt-cache rdepends firefox-locale-en
firefox-locale-en
Reverse Depends:
firefox-testsuite
language-pack-en-base
$ apt-cache show language-pack-en-base
[...]
Recommends: firefox-locale-en
[...]
Description-en: translations for language English
Translation data for all supported packages for:
English
.
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
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