Never heard of Mithra. Sounds mythical. 
You've never heard of Mithraism? It was a pagan mystery religion in the Roman Empire, commonly practiced by soldiers. It originated in Persia, I believe, and "meetings" would revolve around some sort of meals eaten in mithraeums (you can still visit some of these in Italy today). The common depiction of Mithra has him wearing a phrygian cap, I believe holding a bull with one hand and a knife in the other as he is about to kill the bull.
Anyway, since it's a mystery religion, I don't think we know that much about it. So for someone to claim that Christianity was based on Mithraism makes the claim even more suspect.