قُلْنَا يَا نَارُ كُونِي بَرْدًا وَسَلَامًا عَلَىٰ إِبْرَاهِيمَ (We said, "Ofire, be cold and safe for Ibrahim." - Al-Anbiya 21:69 ⧉) As stated above, one explanation of the fire becoming cool and comfortable for Sayyidna Ibrahim (علیہ السلام) could be that it ceased to be fire and changed into a pleasant breeze. However, the more plausible explanation is that the fire remained as fire but did not touch the body of Sayyidna Ibrahim (علیہ السلام) ، although it burnt other things around him, so much so that it even burnt down the rope which bound him.
According to some historic versions he remained in the fire for seven days and used to recall that those seven days were the most comfortable time of his entire life. (Mazhari)