I've never heard about this type of problem before. Pretty wierd. A quick search of google found this. Can you check and see if you have the property dom.screenBrightnessProperty.enabled and if so, is it set to true? You might also check what plugins/add-ons you have installed and whether one of them might be doing this.But still I don't know why my Brightness resets to maximum when I open Firefox or Thunderbird ?
It does not happen if I open other softwares..