Yes it was.
The prophet Zoroaster is supposed to have lived either 1700 BC or 700 BC (the dates are conflicting) in what is now Tajikistan-Uzbekistan (all of Central Asia was part of the iranic cultural and ethnic space before the Turkic invasions of the middle ages.
Zoroastrianism was the religion of Cyrus the Great (Khorush) who freed the Jews and set up the great empire ever - ruling over an estimated 40% of mankind - with the majority of them welcoming his rule.
It was the major religion of Iran before the Arabs took over in the 7th century. Note that Persia/Fars is a small area of south-eastern Iran, colocated with what was Elam. Iran is the older name for the wider area of what is now iran.
The Zoroastrians with the specific denomination of Zurvanism (closer to monotheism than the dualism of earlier Zoroastrianism) was the religion of the Sassanid Empire (c 200 AD to the Arab times) but the secondary important religion was Christianity - specifically Assyrian/Chaldean christianity, which we incorrectly called Nestorian Christianity.
it is guessed that a large minority were Christians and even some of the ruling family.
Islam came and was exceptionally harsh on Zoroastrianism as they are not “people of the Book”
Islam: all of the monotheism, none of the grace and love. The devil wanted to get to Zoroastrians before Christ could.