Spontaneous combustion?
Carbon arc lights such as those used in movie projectors are bright; and hot enough to weld steel.
That it can set celluloid aflame does not particularly trouble me. Temperature that high can also ignite cotton / poly blends, etc.
But whether the trunk of a car would set it off, I'm not sure.

I vaguely recall hearing stories of movies that end up as dust in the storage can. I'm not sure if the idea is the spontaneously ignite, and burn out in there; or if they simply degrade.

Whatever whatever. The notion that it's petrochemical plastics, or nothing is obviously silly.
We won't need a Manhattan Project scale R&D program to invent waxed paper.
"pesticides and gasoline are specialized usages" Red

Very well. If you wish to insert the word "specialized" at the syntactically appropriate place in the following sentence, I'll offer no objection.
"A more dangerous substance is not to be found that was in common usage.

Fiery Red"