Following up (with apologies for the delay; I'm have some troubles with my computer); per
Upholstery in America & Europe from the Seventeenth Century to World War I, edited by Edward S. Cooke (emphasis mine):
Easy chairs were seen in luxurious Paris Apartments by the 1690s. The great sofas of the period were merely double-seated version of these easy chairs. These upholstered forms were at first to be found primarily in bedchambers and grands cabinets... , rather than in the more formal withdrawing rooms and salons.
The great comfort of these late seventeenth-century easy chairs and sofas came with the use of huge, down-filled seat cushions ["squabs"] set in wells formed by padding in the underseat. For padding backs, [curled] horsehair was used because of the ease with with it could be secured in place by stitches. The invalid chair of Philip II of Sapin had a back padded with horsehair, and the bill for some chairs made for Charles II of England in 1660 & 1661 included the item "curled haire to fill the chaire backs." Horsehair became a common back stuffing around 1670, but only in the eighteenth century was horsehair used all over chairs.
The book is a compendium of papers delivered at a 1979 symposium, and remains a standard reference on the topic. For information on 19th century upholstery technology, I highly recommend Katherine Grier's wonderful
Culture & Comfort: People, Parlors and Upholstery 1850-1930; originally published in 1988 in conjunction with a superb exhibit at the Strong Museum in Rochester, NY; it was reissued in the late '90s in a smaller, "university press"-ish format,
sans color illustrations.
