"Is a pastel lesser and a firefly the same and if not, what is it referred to as?"
You asked if a pastel lesser and a fire pastel were the same thing... If you wanted to know if a lesser fire and a butter fire were the same thing you should have asked that. Semantics, I guess.
I consider butter and lesser to be the same morph so yes, I would consider a lesser fire to be a nuclear, as well.