I've no idea what temperature mylar fails at, but I wouldn't be surprised if it's lower than that of a full throttle forest fire.
Typically meant for 300F and below. Melts at just a bit below 500F. Forest fires are several times that temp (like 1600F to 2000F).
This isnt mylar though, mentioned elsewhere its called aluminized structure wrap.
Aside from some of the basics I mentioned about it though, I have no direct experience using it. It was discussed in FF training.