R.V.
I have had some expierience with this and consulted several trainers. I agree with some of the suggestions you have been given. The bed in the closet works pretty well if it will dampen the sound enough. The other thing to try is to distract her with something she really likes. It has to be something REALLY good. When you know the noice is going to start you need to distract her with it. It does take time but she will associate the good thing with the loud noice. Try not to cuddle, pet, etc when she is frecking as this reinforces the "bad behavior". FIgure out how to "snap her out of it", get her mind onto you instead of the noice. If you have a basement, that may be a good area to work/play with her while the fireworks are going off because it should be quieter down there.
She will probalby always react a little but a lot of praise and encouragement with continually lesser reactions will eventually desensitise her to the noice.
Good Luck! If you have trouble, consult a trainer, they can give you more specific ideas and techniques.