If I wanted to mod Fort Firemoth after I finished the quest, perhaps make it into a stronghold, or just a functioning fort and mine again, would I open the Morrowind.esm or the Seige at Firemoth.esp?

Thanks!

Both. Siege at Fort Firemoth is dependent on Morrowind.esm.

What I would do is load Seige at Firemoth.esp as the active file and make direct changes to it. Plugins that make changes to content added by other plugins are always... shaky.

Yeah, I kinda figured this was something not quite in the realm of normal modding.

It just bugs me that the fort is a total waste when it was potential for so much more.