If you look into the FAQ section here...you will see the last thing you should do to a tree in stress is fertilize it.

I'm not familiar with your zone...and this might be a tropical variety. But if the weather is nice having it outdoors is best, water less frequently due to the foliage not there needing the water.

Some bonsai are finicky and can go into shock when moved to a new environment...spider mites also can be troublesome and cause issues. Many things...can cause this.

Typically a scratch test to see if there is green under the bark might be helpful as well.

Because of what you were told, ("it's an Indoor Bonsai") the fate of the plant was already sealed. This is a very, very common occurrence.
You really should take this out of it's pot and plant it in the garden for a year or two and see if it recovers. Truth be known, if it survives, it will be a hard road back. Putting it back into Mother Earth will not harm it, she might even heal it.

Just do the plant a favor and put it in the soil outside, (weather permitting) if it dies, then it was already to late, but if you keep it in that pot and indoors it is dead anyway.

It's not your fault, so don't go beating yourself up. The blame go to the person or persons that sold you the plant in the first place, telling you that it was an "Indoor Bonsai." You see, they care not what happen to the plant after it is sold. Their interest is in the money in your pocket. But from now on it is your responsibility to do what you can to help your Bonsai.

Mark it down as a lesson learned, we are never to old to learn.

Get some good basic Bonsai books and study them, don't just read them. Different authors will give you different slants on the same subject, but that OK.

My Step-father said "if you give advice, plan on being ignored." That has never failed me or stopped me. So here goes.
Just a word of advice, if you have any other "Indoor Bonsai" plants, get them outside also, before it's to late.

How is your Child, I am a proud great grandfather and I love my great granddaughter very much. I thank God that I was given time to meet her. Take care and Good Luck with the Firethorn.

I think, but not a hundred percent sure, Firethorn is a evergreen scrub, therefore it should be outside.