So the Falls themselves just "are" - they can't be open, closed, shut down, or whatnot. There are sidewalks and such that lead to places where you can see them, and the water flows year-round. From a natural point of view, winter brings two important changes: shorter days and more cold.
The shorter days means it's more likely to be dark when you're there, which is cool because they shine coloured lights onto the Falls (they call them Illuminations) to very nice effect.
The cold means that the spray may freeze onto things which can be interesting and fun to look at. The tourist board wants people to come year-round, so they do things like the Festival of Lights to attract you. As a result you can expect the usual restaurants, souvenir shops, and non-Falls attractions to be ready and waiting for your custom.
Journey Behind the Falls is open year-round. So is the Table Rock Welcome Centre and the restaurant in it, Elements. Back when we went, the boat that goes close to the falls was called Maid of the Mist, then it had another name, and now it's Hornblower Cruises but anyway it stops running Nov 30 so it's not an option in the winter. That seems to be about the only thing you won't be able to do year-round, but since you intend to come in mid-November, even that is an option for you.
Dress warm, embrace the dark, and prepare for a magical experience!