Hardwood

Considering you need a diamond coated blade to cut this hardwood it's probably not good firewood material.

The wood in the Petrified Forest is actually from the Late Triassic Period(225 million years ago).
Over time, trees died or perhaps were knocked over by floodwaters or wind. Rivers carried the trees into the lowlands, breaking off branches, bark, and small roots along the way. Some trees were deposited on the flood plain adjacent to the rivers and others were buried in the streams and channels. Most of the trees decomposed and disappeared. But a few trees were petrified, becoming the beautiful fossilized logs we see today. Most of the fossilized logs are from a tree called Araucarioxylon arizonicum. Two others, Woodworthia and Schilderia, occur in small quantities in the northern part of the park. All 3 species are now extinct.

Some logs were buried by sediment before they could decompose while volcanoes to the west spewed tons of ash into the atmosphere. Winds carried ash into the area where it was incorporated into the deepening layers of sediment. Ground water dissolved silica from the volcanic ash and carried it through the logs. This solution filled, or replaced cell walls, crystallizing as the mineral quartz. The process was often so exact that replacement left a fossil that shows every detail of the logs’ original surfaces and, occasionally, the internal cell structures. Iron rich minerals combined with quartz during the petrification process, creating the brilliant rainbow of colors.

Info provided by: http://www.petrified.forest.national-park.com/