Cold? Very cold? Not even......
Winter is the most spectacular time of year to see the Falls, IF there is snow...which this past winter there was barely any of. Yes, Niagara lies in a snow belt, but does not actually see much snow. We just happen to get a few freak ice storms a year. The Falls is gorgeous during the winter, especially if there is ice on all the trees and everything is crisp. Add in the bonus of less tourists. The main attractions will still be open, the restaurants....just the more carnivalesque ones close up for the winter. The one downside is that the Maid of the Mist does not operate during the winter. New Years is a great time to be here. Concerts at the Falls, fireworks, food, the Festival of Lights.
That being said, November is not winter yet....nice Autumn weather. Perhaps the occasional snow flurry that doesn't stick and melts as it hits the ground. Temps usually hovering in the + 5 range. December you will get a bit more snow that might stick for a day or two here and there. Temps around the -2 to +1 range. January is when winter really hits. Temps in the -5 to 0 range. February, about the same. March, the snow melts, although be prepared for quick weather changes....snow one day, warm weather the next. Temps all over the spectrum. April is generally more rainy, and temps are still all over the place. This year we had +20's in mid March and ridiculously warm weather allllll spring. I've already had my CA on quite a few days.
Just keep in mind Southern Ontario winters tend to be of the wet and sloppy variety. Tons of slush.....plus the mist from the Falls makes everything close by wet too. A good waterproof jacket will be required. Gloves, hat, scarf. Good walking shoes. Enjoy!