As Muslims we believe that all people are born with the natural inclination to worship God; One God, nor a plethora of gods and demi gods.  Therefore it makes perfect sense that in the ancient world, including the Persian Empire there were people who worshipped One God. Islam completes and purifies such monotheism.

It has been said that Zoroaster was pious, noble and compassionate.  He preached that a moral life paved the way to eternal bliss.  He encouraged his followers to love the righteous and have compassion for the distressed.

Like Islam, Zoroastrianism does not discriminate between men and women, both sexes are treated equally in the religious texts. There is no preferential treatment of male children and children are advised to honour both father and mother equally.    

What exactly does Quran have to say about these monotheistic religions that pre date the Islam we know today? Who are the people mentioned along with Jews and Christians?

Verily, those who believe (in God and in His Messenger Muhammad), and those who are Jews, and the Sabians, and the Christians, and the Magians, and those who worship others besides God, truly, God will judge between them on the Day of Resurrection.  Verily! God is over all things a Witness. (Quran 22:17)

Zoroaster had a great reputation as a scholar and physician.. He established many fire temples and suppressed the worship of demons, and the use of magic, witchcraft and sorcery.[1]  Zoroastrians were indeed the people mentioned in Quran as Magians

Although the ancient Greeks viewed Zoroaster to be the founder of the religion of the Persians, they embellished his story to such an extent that a new and novel existence was created for Zoroaster.  He was placed in an impossibly ancient past, some 6 or 7 millennia before the Common Era and was invariably cast an either a King and or a sage.  Zoroaster began to be portrayed not only as a sorcerer and astrologer but as the inventor of magic.  The Greek love of exotic wisdom and the evolution of the word mage from learned one to practitioner of magic were combined to incorrectly define Zoroaster as a magician and the Zoroastrian’s as Magians ( def.  magicians).

The origin of the word magi in Persian sources is unclear but its use in Zoroastrian scriptures seems to refer to a member of a tribe or group, possibly leading to the word magian coming to denote Zoroastrians.  In Christian literature the three men who travel from the East to meet the new born Jesus are referred to as the magi or wise men. 

The origin and meaning of the word Sabian (mentioned in Quran with the word magian) is also shrouded in mystery however we can be certain that Magians and Sabians were two distinctly different religious groups

The original Sabians did not worship either the angels or the stars but are said to have followed the teachings of Prophet Abraham. In short their beliefs and monotheism was similar to that of the Jews, however they came before the Jews possibly as far back as the time of Prophet Noah.  Sabians are sometimes referred to as the people of Prophet Abraham just as the Jews are called the people of Prophet Moses[2].   The four religious groups mentioned in Quran, Jews, Sabians, Christians and Magians all practised forms of monotheism, sometimes adulterated, thus not always the pure monotheism of Islam.

Thus, in conclusion we find that the Zoroastrians were never fire worshippers, magicians, sorcerers or astrologists.   At one time Zoroastrianism dominated an area stretching from what is now Rome and Greece to India and Russia.[3] Their numbers dropped when the Persian Empire was conquered in the 7^th century C.E. 

Zoroastrians do not preach their religion; in fact, many modern Zoroastrians claim that conversion to their religion is impossible.   While there are priests there is no hierarchy and no intermediaries between the people and God.  The basic doctrine of Zoroastrianism revolves around  good thoughts, good words, and good deeds.[4]

[1] http://www.dlshq.org/saints/zoroaster.htm

[2] Ibnul Qayyim in Ahkaam Ahl al-Dhimmah, 1/92-98

[3] (http://select.nytimes.com/search/restricted/article?res=F40E16FB3D550C758CDDA00894DE404482)

[4] http://www.nytimes.com/2006/09/06/world/americas/06iht-faith.2713551.html?pagewanted=all