i dont think firs take to trimming basicly but that is a guess im sure someone will tell you better than i can .

This sounds like a cupressus hedge rather than a fir. The tips do tend to go brown if they are cut back a bit too close but they generally recover quite quickly in the growing season.. The bare brown patches could be a disease that lots of people have noticed and I don't think there is a cure - maybe someone else will know more about this than I do.