So would you be driving to Montreal then flying out of there? Or flying from somewhere in New York to Montreal, then connecting to Orlando?
I'm be sceptical on getting better deals direct out of Canada - our airfare often much higher than US domestic flights and a lot of people close to the border will drive to the US to take flights, but if the math works out, it may be worth it. Clearing customs in Montreal could be anywhere from a short 5/10 minutes, up to maybe an hour (not sure as I've only cleared US customs in Canada in Calgary) depending on time of day and how many other flights there are. If you are driving to Montreal, then flying, I'd be potentially prepared for some extra questioning on why you are flying out of Canada. Also if that is the case, then I'd really figure out what additional costs there would be for transport/parking etc.