I am apt to suspect the negroes and in general all the other species of men (for there are four or five different kinds) to be naturally inferior to the whites. There never was a civilized nation of any other complexion than white, nor even any individual eminent either in action or speculation. No ingenious manufactures amongst them, no arts, no sciences. On the other hand, the most rude and barbarous of the whites, such as the ancient Germans, the present Tartars, have still something eminent about them, in their valour, form of government, or some other particular. Such a uniform and constant difference could not happen, in so many countries and ages, if nature had not made an original distinction betwixt these breeds of men. Not to mention our colonies, there are Negroe slaves dispersed all over Europe, of which none ever discovered any symptoms of ingenuity; tho' low people, without education, will start up amongst us, and distinguish themselves in every profession. In Jamaica indeed they talk of one negroe as a man of parts and learning; but ‘tis likely he is admired for very slender accomplishments, like a parrot, who speaks a few words plainly.
Of this footnote the article says,
This should be understood in its historical context, of course, such views were all but ubiquitous in the intellectual establishment (as elsewhere) of the time, and indeed would continue to be for a century after his death. Unlike many others of his day and much in advance of his time, in 1758, Hume condemned slavery at great length.
The claim that such views were all but ubiquitous in the intellectual establishment of the time is, if it refers to eighteenth century Scotland, much more complicated. One of the early responses to the footnote was by his much more popular contemporary, James Beattie, who tears it to shreds. Beattie is an egalitarian; he believes that all men are created equal, and he attacks, mocks, and refutes every point here. The Internet Encyclopedia of Philosophy has a lovely summary of some of Beattie's main points. Hume was not in advance of his time; his condemnation of slavery in the essay on the populousness of ancient nations (to which the 1758 date refers) is far more restrained and ambiguous than condemnations that already existed. Gershom Carmichael had attacked the institution far more fiercely in his work on natural rights (PDF). People like Beattie and Carmichael were ahead of their time; Hume was right in step. The short passage that the Wikipedia article calls a condemnation "at great length" amounts to a criticism of slavery as a way of breeding antisocial habits in the masters. How unimpressive this is can be seen in comparing it to Carmichael's or Beattie's attack on it as inconsistent with natural rights. From the end of Beattie's long attack on Hume's racism:
It is easy to see, with what views some modern authors throw out these hints to prove the natural inferiority of negroes. But let every friend to humanity pray, that they may be disappointed. Britons are famous for generosity; a virtue in which it is easy for them to excel both the Romans and the Greeks.
Let it never be said, that slavery is countenanced by the bravest and most generous people on earth; by a people who are animated with that heroic passion, the love of liberty, beyond all nations ancient or modern; and the fame of whose toilsome, but unwearied perseverance, in vindicating, at the expense of life and fortune, the sacred rights of mankind, will strike terror into the hearts of sycophants and tyrants, and excite the admiration and gratitude of all good men, to the latest posterity.
[James Beattie: Selected Philosophical Writings, James A. Harris, ed. Imprint Academic (Charlottesville, VA: 2004) 137.]
Eric Morton discusses the infamous footnote at some length in his important article, Race and Racism in the Works of David Hume.