Is this door prehung or just the slab?

It is listed as a single door slab with a left-hand hang, not a prehung unit. You supply the frame, hinges and lockset, or have a contractor fit it.

Can I paint a Smooth Pro fiberglass door?

Yes. The smooth white surface is designed to be painted, and fiberglass holds exterior paint well. Clean and lightly scuff the surface first, then use an exterior-grade paint.

Does it come with glass or a window?

No. This model has no glass, so the entry stays solid and private. If you want light, look for a Smooth Pro model with a lite or sidelight.

What size opening does a 36 x 80 door fit?

It fits a rough opening sized for a standard 36-inch-wide, 80-inch-tall door, typically about 38 x 82 inches. Measure your opening before ordering.

Is fiberglass better than steel for a front door?

Fiberglass will not rust and resists dents and moisture better than steel, which makes it a good pick in humid or coastal areas. Steel is usually cheaper and can feel stiffer.