I really doubt zidane will quit Real Madrid even if they lost the Champions Leagues. And I doubt he will be fired from coaching because look at Madrid now. It really has improved thanks to Zidane. Barcelona used to win every game against Real Madrid. Yet, look at it now, either draw or Madrid wins. Added to that, it has been through the finals many times and especially when Zidane was coaching them. So i really doubt that.
Madrid owners is typical people who will not hesitate to sack the manager if the results not in accordance with their expectations although those managers already give plenty of trophy for them and currently only champion league trophy can save Zidane and if he failure to get it i think speculations between his future will appears because i'm pretty sure he won't comfortable anymore with his position