Is plastic sheeting flame resistant? No... Can plastic sheeting be flame resistant? Again, no. Not if you adhere to the definition that differentiates flame resistant and flame retardant, or fire retardant.
It's a bit tricky because more often than not, flame resistant and flame retardant are used interchangeably. Here is the minute difference.
Flame resistant fabrics (notice the word "plastic" is missing here) are fabrics that are inherently flame resistant by their chemical nature. If a fire is lit under a flame resistant fabric, it will prevent the spread of fire. They may burn, but they will burn slowly, and often self-extinguish.
Fire or flame retardant plastic (plastic sheeting) and fibers, are items which by themselves will burn if a flame is placed on it. What makes some fabrics and plastics fire retardant is the special fire retardant chemicals that are either added to the polymer while it is being formulated, or treated with these chemicals, which then makes the fabric or plastic fire retardant.
Plastic sheeting made from polyethylene will burn big time when hit by flames. The only saving grace to slow the fire is if the polyethylene is treated with a fire retardant chemical. Since plastic/ polyethylene is made from oil, it can really make a fire become a big fire in a hurry it ti's not treated with and FR additive.
Better safe than sorry- FR plastic is the way to go to be safe.