Hello, Polex, and welcome to MLS!
I have seen a product specifically for what you are talking about, but can't remember the manufacturer. I will try to find it, but I responded to this now, because I'm hoping someone else will see your question and have a better memory than myself!/DesktopModules/NTForums/themes/mls/emoticons/blush.gif
As far as I know, Hillside does not exist any more. I have one of their kits in my LGB Forney and it works pretty well. The LEDs are bright and if you drill a hole in the bottom of the firebox, the light will shine down onto the tracks which at night, looks pretty cool. The units in the LGB moguls are neat because they turn on and off simulating the firedoor being opened and closed. Now that I recall, someone did an article in GR that involved using a mini transistor radio to make caboose marker lights flicker. I'm guessing that that can be adapted into a flickering firebox too.
Personally I think that firebox glow is not realistic. The only time there is a firebox glow is when the firebox door is open. Unless the engine is really really hard working hand fired engine where the fireman is continually shoveling coal, most of the time there is no glow from the firebox since the door is closed.
My personal experience was that operating a hard working steam engine at night it wasn't so much a glow as a blinding light when that door was open. The fire could be pretty intense and while running the engine it was important to not stare over at the open fire when the door was open. Keep your eyes on the tracks ahead.
Maybe you didn't know, but if you put a Sierra sound system in the engine, it includes the firebox glow feature and all you have to do is mount a couple diodes in the firebox area. (Just in case you were thinking of getting a sound system too.