Being a little suspicious of the "pyre"-fire connection, I looked "pyramid" up in several etymological dictionaries.
Etymology online says this:
1552 (earlier in L. form piramis, 1398), from Fr. pyramide (O.Fr. piramide, 12c.), from L. pyramides, pl. of pyramis "one of the pyramids of Egypt," from Gk. pyramis (pl. pyramides), apparently an alteration of Egyptian pimar "pyramid."
"Pyramis" in Greek is different from "pyra", which means "a hearth, funeral pyre". Funeral pyres weren't called "pryamis" nor was "pyramis" applied to mountains except to describe the shapes.
A Spanish etymology site adds "pyramis originally stood for a kind of wheat flour cake in the shape of a pyramid, and that pyros meant wheat flour." [
forum.wordreference.com]
That "sort of" makes a lot of sense, because I know that offering loaves were somewhat conical in shape. I'm not so sure about how solid the "wheat flour" connection is. To the best of my knowledge, the word comes into Greek about the time of Herodotus and was used to mean "the shape of that big thing out there next to Cairo. It is not a word from great antiquity, used to denote mountains ... or volcanoes.