Is fibreglass dangerous?
Fiberglass insulation (the pink fluffy stuff) is a skin irritant and not great to breath either.  Keeping it in thick plastic garbage bags would be ok - unless the bags rip it you use really thin plastic bags.

A better idea would be styrofoam insulation.  It comes in big sheets you could cut to wedge perfectly in a window frame.  It does not have the same problems of being an irritant.