Zoroastrians believe in Mazda, the creator god. Water and fire are considered agents of purity. A sacred fire is housed in a fire temple, such as the one in Yazd. This one is supposed to have been burning for hundreds of years, and moved around many times, until it was set up in this temple.
They practiced a form of sky burial until the 20th century. Clothes were taken off a body, which was then left exposed to the sky on a rounded mountain top. After the scavengers such as vultures had stripped the flesh off the bones, the bones were thrown into a very deep pit in the middle of the site.
This was done, presumably, so that the rotting flesh do not defile the scared earth. It can, I suppose, be considered environmentally friendly. Even though it may sound gruesome and disturbing.
Iran is dominated now by Shiite Muslims. Yet Zoroastrian, as well as Armenian Christian communities survive, openly. One of my greatest rewards of this trip to Iran is seeing at first hand how this is happening.