The Fire and Rescue Service has been called to a number of fires in recent years where the most likely cause was sunlight reflecting off a mirror.
Where you have a mirror or any other glass object, such as an ornament, that is curved, this allows the glass to act as a lens. When the sunlight hits it, it becomes concentrated and, if the rays are bounced onto something flammable (such as curtains or soft furnishings) then a fire can start.
The principle is the same as using a magnifying glass to start a fire. It does depend on the angle of the sun, but any mirror or glass ornament with a curve in it should never be left on a windowsill or by a window.