As Jura points out, there is no essential difference. I have looked up froh and fröhlich in the dictionary, when using the words I found, I would translate "Frohe Weihnachten" = "Merry Christmas" "Fröhliche Weihnachten" = "Joyful Christmas" which again, is more or less the same on my mind

There is no difference between Frohe Weihnachten and Fröhliche Weihnachten , either is acceptable.