I've uninstalled Firefox, but keep getting English language pack for Firefox offered to me via the updater. Is there a way to make sure this never happens again?
I just checked languages under system settings and applied English (United Kingdom) system-wide. Will that do it? If so, I'll leave this here to possibly help others in the future.
For some reason, the language packs are separate packages that do not get uninstalled if you uninstall Firefox. However, you can safely uninstall the language pack.
sudo apt-get remove firefox-locale-[your version].
If removing this would happen to remove other packages that depend on the language pack, it will tell you so and you can cancel the removal. But I'm quite sure it won't.