Is an electric dryer's vent pipe a fire hazard when routed through the floor?
It's not a concern. Dryer exhaust might hit 200 degrees, but most have thermal fuses far below that. You'd need twice that and then some to start a fire. Heck, many dryers are vented through cheap vinyl hose. If it was a concern building codes would require double-wall flues akin to those used for gas appliance exhaust. 

I do recommend against the lightweight foil hose, though. It really accumulates lint. Use semi-rigid aluminum pipe at least (example). Its ridges are much smaller and it's less likely to close up on itself in a bend. 

You may have heard of the risk of not cleaning the lint filter. This can result in reduced airflow and heat accumulation, but any fire is likely to begin in the dryer in this case. More on that