Nitrate film off-gases nitric acid and is extremely flammable. However, the tales of early films spontaneously combusting are mostly urban legends. They are, however, extremely flammable and can even pose a health risk to archivists and preservationists, as well as a preservation risk to other archival materials. I have seen information in books where nitrate films have disintegrated in metal storage containers, completely rusting out the inside from the nitric acid off-gas.
Because of the instability of the nitric acid film base, and given the alleged age of this film, we are truly blessed to be able to see it. Note that the man who appears to be an Orthodox monk in the screenshot above is actually an "Armenian Pope".