Canada, being a British territory and having its foreign affairs dictated by London, was a hotbed for Confederate agents and sympathizers who could operate north of the Union's borders - Montreal in particular had a sizable cell that launched a cavalry raid on Vermont that accomplished... nothing, one person was killed. Halifax at one point offered amnesty to a CSA-Navy ship being pursued by the USA Navy and basically passively aided one nation against the other. There was also a bizarre episode where Confederate agents hijacked a ship, but when the Union arrived, they allowed it to be returned to Halifax instead of taking it to Union waters or attacking it, and the colonial courts ended up acquitting the hijackers and never extraditing them or anything. The Union was cooperative in order to preserve diplomatic relations. Canada's relations with the two American factions is complicated and paradoxical.
It's true though that at least 33,000 British North-Americans served in the US Army during the Civil War of their own free-will. The basic opposition to slavery overpowered for many the purely self-serving British sympathies for the CSA. Supposedly at least 20 of these volunteers earned the Medal of Honor, but I have no idea how to confirm that.