Probably best to sudo dpkg --remove firefox-locale-en
Be forwarned that if you intended on managing software on your machine this way, many of the tools assume understanding of package management, dependencies etc and may generate warnings and offer override options. The overrides are provided assuming the user is fully aware of the ramifications. The overrides are there for specific reasons, they aren't there as a 'suggestion' on what to do.
Doesn't LM have a software management tool? Why can't firefox be removed there?