I am an Indian citizen traveling from Nice (France) to Boston (USA) via Montreal (Canada). My layover in Montreal is for 2 hours. I already have a multiple entry visitor visa for Canada, do I need a transit visa?
You can transit Canada using any type of temporary resident visa (tourist, business, student, etc). A separate transit visa is not required.
Note that on this itinerary you won't pass through Canada immigration anyway; you will go directly through the transit corridor to US preclearance. (This doesn't apply if you have separate tickets or your checked baggage is not checked all the way through for some reason. In that case you'll have to go through Canada immigration, pick up your baggage and recheck it. But two hours likely isn't enough time for a connection on separate tickets anyway.)
(Also, other readers without any Canadian visa should note that Canada's transit without visa program is limited to only a few nationalities; most people will require some sort of visa or visa-free access to Canada to transit even if they will not need to pass Canada immigration.)