My father immigrated to France from Poland in 1973 with his family when he was 13 and became a French citizen. He them came to the U.S. in 1978 and became a U.S. citizen in the late 90's. He was a dual citizen of both France and the U.S. all of these years. Recently, he passed away. All of his family is in France. Could I become a French citizen as his son, since he was a French citizen? I was born in the U.S. and my mother is American.
You almost certainly are a French citizen already (and if you are, it would be because you have been one since your birth). If you wish to use that citizenship and perhaps pass it on to your children, you will need to apply for a passport, ID card and/or a certificat de nationalité française at the nearest consulate.
I would advise doing that now rather than later because there is a 50-year clock that may cause people living abroad to lose their French citizenship under some conditions (I don't think it would apply to you in this case but it might apply to your – current or future – children and since your father left France in 1978, it has not been 50 years yet so that applying for a French ID now would make the whole issue moot).
In particular, the certificat de nationalité française is the strongest form of evidence of French citizenship and would be a useful document to have if you or your descendants ever want to make use of that citizenship. However, it does not replace an ID if you want to vote, travel as French citizen, become a resident in another EU country, etc.