I think it should be on its own mask under the furniture and under the shadows they cast, right?

There are numerous ways of doing it, but the best way is to have 2-3 c. 50% opacity overlay layers of pale reddish-orange or whatever colour you want. For each layer, take a HUGE very fuzzy brush and scribble around your forges - each of the 2-3 layers you scribble a slightly wider area around each forge. Then blur each layer up with the blur tool to avoid hard edges. The piling up like this of three hand-drawn fuzzy overlay layers will create the illusion of hot red light fading from the forge to the outer walls. Then you select you shadows (alphas to selection) and delete that part of each layer, then do the same for the furniture - but then you have to make sure to draw a NEW set of reddish forge-heat reflections over the top of the furniture, just like we discussed above.

The aura of light should be circular if the wall around the forges are circular. If the wall has a lower opening to the front, it might not be circular.