how hot and dense are fireflies?

Fireflies are very hot indeed. They can reach temperatures of several million degrees. That's much hotter than anything that occurs naturally on Earth.

... and how dense are they?
Much less dense than the air we breathe is the short answer - by a long way!  X-ray bright points are one hundred million times less dense than the air we breathe. Even so, they are more dense than their surroundings in the Sun's atmosphere.