I can't speak for the other provinces, but the border between Newfoundland and Labrador and Quebec was settled by a 1927 decision of the Judicial Committee of the Privy Council (then effectively the court of final appeal for Canada and other Dominions - including Newfoundland, which didn't join Canada until 1949).

Before that, Quebec claimed much of what is now Labrador (and some provincial maps still show Labrador as part of Quebec, or at least refer to the boundary as "disputed," even though no one else disputes it).