I have recently gotten really into this... I'm not a professional historian or anything, but I'm a Canadian with a love of history. The first African slave recorded in what would become Canada was Olivier le Jeune in 1628. He was a boy of about 8 years old. This was in New France. Slavery was, at the time, not legal in France, but the colonists got special permission from France to practice slavery in New France. By 1759 there were approximately 4000 slaves in New France, more than half of them actually being First Nations, not black. The biggest influx of slaves (African slaves) came with the British loyalists who migrated to British North America (which would also eventually be Canada) from America around the time of the American Revolution...they brought about 2000 slaves with them.

But you are right, slavery never took off in Canada. This has nothing really to do with any moral superiority, it's the same reason it didn't take off in the northern states...the climate. There were no plantations. During planting and harvests, having a lot of slaves would have been helpful, but for the rest of the year, they would have just been extra mouths to feed. This meant that while in the South, big strong men were prized and more expensive, in Canada, women (who were seen as more suited for domestic work) were more highly sought.

There is also the fact that the laws governing the treatment of slaves were quite different. French colonists were catholic, and the Catholic Church neither endorsed nor condemned slavery, but it did give very specific (and to my mind, contradictory) laws regarding the treatment of slaves (contradictory, because these rules were in place because the Catholic Church acknowledged that slaves were people with souls, but even with this knowledge, it was somehow perfectly fine to enslave them). These rules included things like not physically mistreating them (aside from deserved punishment that would be decided in a court), not separating families (marriages between slaves were legally recognized), etc.

One of the strangest aspects to Canadian slavery as well is that, from the time of the American Revolution onward, American slaves were able to escape into Canada and be free. Some of them served in the British army in exchange for their freedom, but most of them just...came. (It appears this was just done to undermine the US economy). So you had free black people living in the same country, under the same government, as slaves. It's bizarre. And went a long way in undermining the entire institution.

In 1793, Lieutenant Governor John Graves Simcoe and Peter Martin, a free black man, witnessed a slave named Chloe Cooley being abused as she was thrown into a boat to be sold across the Niagara River as she screamed in protest. This upset them so much that they immediately began to push for legislation against slavery. They were met with strong opposition, and were forced to compromise with legislation that gradually phased out slavery within one generation – anyone currently a slave would remain so until they died, their children would be free at age 25, and their grandchildren would be immediately free. Simcoe was personally disappointed with the legislation, he did not feel the law went far enough in freeing those who were currently enslaved. However it was the first law limiting slavery at all in the entire British Empire.

Leaving another comment just to amend, or sort of soften the tone maybe, or something I said in my previous comment. I mentioned that the reason slavery never took off in Canada was not due to any moral superiority, and I firmly believe that -- I believe if you switched the colonists from Canada and the Northern US with the Southern colonists you would have very similar results -- the conditions in the South just made it very beneficial for them to blind themselves to what they were doing. But well, that doesn't mean that there weren't people in Canada (and the Northern states) that weren't firmly, personally convicted that slavery was a moral evil. I think some of my bitterness at having the fact that slavery was practiced in Canada at all hidden from me came through in that sort of off hand remark -- I found out when I went to write a paper on why Canada had never practiced slavery and just sort of accidentally found out that oops... we did. I'd never been taught about it, and when I brought it up with my history teachers, none of them even knew about it.

So yeah, there were many Canadians (white and black) who did fight against the institution, and even those who spent significant amounts of money to buy land to give slaves escaping to Canada a place to settle (there is still a small city in Canada today that was founded on this money that Canadian abolitionists just sort of handed over out of principle). So the stories of the Underground Railroad and the part Canada played in that are true, but... well, I just believe the whole story should be told.

This question was asked only yesterday, actually, so you might find the answer given in that thread by /u/ShadowsSheddingSkin to be of interest.