No because, Firn and Harsch are words on their own.

Yes, because of the way the German language works. It tends to create new words by combining old words not by creating new short words (Dialects like Bavarian work differently though, they often tend to create new words).

Then after centuries people forget that and think it's one word. Like "Enttäuschung" (disappointment) which people no longer realize what the two words are and that "Enttäuschung" really means that you had been deceived ("Täuschung") and now are not longer - the deeper meaning of "Enttäuschung" in German. Same for "Werkzeug" (Tool) - the words get their own identity.

What I found most interesting was Rücksicht, Vorsicht, Nachsicht, Einsicht, Weitsicht (and more) where probably no German would think they are the same word, "Sicht" combined with another one. All of those words have their own, distinctive identity.