Suggested source for the fiery furnace legend

The miraculous deliverance of Abraham from the fiery furnace is recorded in Beraishis Raba and some other sources. It is not in the text of the Torah itself (though a friend of mine once spent an afternoon searching for it).  Berashis Raba is a late work and an anthology of earlier material. What is the origin of the fiery furnace story?

Torah True Answer 
It actually happened and the event was recorded in the collective memory of the Jewish people, until at some point someone, for some reason, decided to write it down. (The earliest record of the story is Philo (I think) It is not in Josephus or Jubilees, two books where early drafts of legends known to us from later Rabbinic works are often found. The Talmud, in at least four places, seems to take for granted the historicity of the event.)

DovBear's answer
Genesis 15:7 says:

וַיֹּ֖אמֶר אֵלָ֑יו אֲנִ֣י יְהוָ֗ה אֲשֶׁ֤ר הֹוצֵאתִ֙יךָ֙ מֵא֣וּר כַּשְׂדִּ֔ים לָ֧תֶת לְךָ֛ אֶת־ הָאָ֥רֶץ הַזֹּ֖את לְרִשְׁתָּֽה
I am YKVK who took you out from Ur Casdim and gave you this land to inherit

The translation I've provided seems straightforward enough, but what if you didn't know about the ancient city called Ur? How would you interpret this verse if you'd never heard of that place?

As it happens the word א֣וּר appears in the Bible six time, aside from the five other times its used in connection with Abraham's adventures.  In all six of those non-Abraham instances the word means furnace, or fire.

Isaiah 24:15
BIB: עַל־ כֵּ֥ן בָּאֻרִ֖ים כַּבְּד֣וּ יְהוָ֑ה
KJV: ye the LORD in the fires, [even] the name
INT: and in the fires glorify the LORD 

Isaiah 31:9
BIB: יְהוָ֗ה אֲשֶׁר־ א֥וּר לוֹ֙ בְּצִיּ֔וֹן
NAS: whose fire is in Zion
KJV: the LORD, whose fire [is] in Zion,
INT: God whose fire Zion furnace 

Isaiah 44:16
BIB: חַמּוֹתִ֖י רָאִ֥יתִי אֽוּר׃
NAS: I am warm, I have seen the fire.
KJV: I am warm, I have seen the fire:
INT: I am warm have seen the fire 

Isaiah 47:14
BIB: גַּחֶ֣לֶת לַחְמָ֔ם א֖וּר לָשֶׁ֥בֶת נֶגְדּֽוֹ׃
NAS: to warm by [Nor] a fire to sit
KJV: to warm at, [nor] fire to sit
INT: coal to warm by a fire to sit before 

Isaiah 50:11
BIB: זִיק֑וֹת לְכ֣וּ ׀ בְּא֣וּר אֶשְׁכֶ֗ם וּבְזִיקוֹת֙
NAS: Walk in the light of your fire
KJV: walk in the light of your fire,
INT: firebrands Walk the light of your fire the brands 

Ezekiel 5:2
BIB: שְׁלִשִׁ֗ית בָּא֤וּר תַּבְעִיר֙ בְּת֣וֹךְ
NAS: you shall burn in the fire at the center
KJV: Thou shalt burn with fire a third part
INT: One the fire shall burn the center

It appears to me that the legend of Abraham and the furnace may have been built on a misunderstanding. Not ever having heard about the city called Ur, perhaps the ancient interpreters read  Genesis 15:7 in light of the usage found  in  Isaiah and Ezekiel yielding:

I am YKVK who took you out of the FIRE of Kasdim. 

Support for this suggestion comes from many places:

1. TPJ translates the words Ur Kasdim as "the fiery furnace of Kasdim"
2. Pirke De Rabbi Eliezer reports the legend and gives Genesis 15:7 as the proof-text, suggesting that the author of this book, like the author of TPJ,  imagined the word Ur referred to a furnace and not a place
3. LXX translates Ur Kasdim as "the land of the Chaldees" No knowledge of any city called Ur is indicated.
4. The Talmud frequently refers to a place called Kasdim. This is Chaldea, a region in southern Babylon. To the best of my knowledge, the Talmud does not ever speak of a place called Ur. This supports the notion that Kasdim was recognized as a place but Ur was not. This misunderstanding is what would have produced the reading I've suggested above.

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