The Medrash (Rabba 63:2) says that Avrohom was only saved from the Kivshan HaEish in the Zchus of Yaakov as the pasuk in Yeshaya (29:22) says, "Bais Yaakov Asher Padah Es Avrohom." What does this mean?
Rav Shimon Schwab answers that Avrohom's great middah was Chesed. The problem with Chesed is that it totally disregards justification. It is not deserved kindness but rather kindness with no limits and no calculation. This is very dangerous and can have terrible ramifications. The Torah calls Znus, Chesed because it comes from a mistaken desire to be kind. Avrohom's middah could not exist in the world on its own. It only has a kiyum if tempered with Emes, truth. This keeps kindness in check and controls it from being applied wrongly. Emes is the middah of Yaakov. Only because of Yaakov who would eventually combine these two Middos, was Avrohom worthy to be miraculously rescued from the fires.
Rav Schwab gives a second answer as well. Haran also jumped into the fire after seeing Avrohom's salvation. The reason he died was because he jumped in expecting to be saved. That is not true Mesiras Nefesh and because he did not display real Mesiras Nefesh he had no Zchus with which to be saved. Avrohom however jumped in willing and expecting to die. With this Emeseh Mesiras Nefesh he was saved. This is what the Medrash means that Avrohom was saved in the zchus of Yaakov, in the zchus of his emes in jumping into the fire with pure intentions.