Who was Zarathustra? Why is the fire sacred for his followers?

Zarathustra, Persian Azardust (Azar=Fire & Dust=Friend), was born in the Assyrian/Chaldean city of Urmia in Azarbaijan (Land of Fire).

Look also at this thread: Is there any similarity between Islam and Zoroastrianism?

Answers of those questions:

http://answering-islam.org.uk/Index/A/abraham.html

In Islam, Abraham is also called the Friend of God, Khalilu 'llah and his name, Ibrahim, occurs 69 times in the Qur'an, making him the second most-occuring name of prophets in the Qur'an after Moses. He was supposed to be one of the six prophets in Islam to whom God delivered special laws. He was said to be given twenty portions (suhifah) of scripture.
The father of Abraham in the Qur'an is Azar, in contrast to Terah in the Bible (Genesis 11:25-26) and the Midrash. In fact, ibn Hisham gave the name as Tarih, sufficiently close to the Bible's. Thus, this name is known to Muslims. It is a curious question on what basis and for what reasons is his name called Azar in the Qur'an? It is not clear if there is any etymological connection between these two names. However, according to Tisdall on the authority of the historian Eusebius, the Syrian name had a similar sound to Azar. Perhaps, that is how Muhammad came to know of this name during one of his merchantile expeditions to Syria.

"I am the Lord who brought you from Ur of the Chaldeans". (Genesis 15:7)
Ur was a place archaelogy has proved existed, while "Or" means fire.

Nimrod, son of Cush, king over the country ... who took Father Abraham, and cast him into a fierce fire; but the fire grew cool and pleasant unto Abraham, who came out of it after some days. And thereupon his people believed in him.

Originally posted by Cyrus Shahmiri Azarbaijan (Land of Fire).

I'm not sure how you get to that. Azar means Fire, but "baijan" means land? don't think so

Zarat -> Zard (Yellow); it also means old
Hushtra; one who carries/bears

I've read smwhere Zoroaster was in fact Abraham, the prophet who bore fire!!.

right, then Patgan should be Padegan. Now it makes sense Arabs don't pronounce P and G so they changed it to Baijan.

Yes, "Ushtra" exactly means "Camel" and Zarathushtra can also means "Golden Camel" but in the Khurda Avesta, his name is Zarduh�t.

It is not clear when Zarathustra was born but it can't be earlier than 19th century BC, as I said in this thread, Zoroastrianism has probabley more than 4,000 years old.

I remember an article in Encyclopedia Britannica that gave the years 628-551 B.C. for the lifetime of Zoroaster.  This is based on the assumption that Vishtaspa, his most famous convert, was the father of King Darius I.  If those dates are true, Zoroaster's birth and death correspond almost exactly with the beginning and end of the Median kingdom.

As for Azerbaijan, I understand that came from Atropates, the satrap of the area in the days of Alexander.  Afterwards northwestern Iran was called Atropatene, which was corrupted through translations in many languages to give us modern Azerbaijan.

It seem Zoroastrians themselves believe that Zoroaster was born in Takht Suleyman of Iran's Azarbaijan in 3,600 BC!! -> http://www.payvand.com/news/05/apr/1002.html