as @joe Bloggs already mention leather, i suggest boiled leather armor with extra diping or soaked the boiled leather armor with urine and vinegar just extra case because this help extinguished greek fire (and hopefully the urine can cool down the heat) according to wiki link here:
In attempting to reconstruct the Greek fire system, the concrete evidence, as it emerges from the contemporary literary references, provides the following characteristics:
It burned on water, and, according to some interpretations, was
ignited by water. In addition, as numerous writers testify, it could
be extinguished only by a few substances, such as sand (which deprived
it of oxygen), strong vinegar, or old urine, presumably by some sort
of chemical reaction
and the best part is you can make boiled leather armor (not the rock star leather jacket that you think of but real practical hard leather armor) using boiled urine, urine is part of traditional leather process anyway.