It can take as long as 20 seconds to scroll the screen, which tends to scroll in large jumps, even when smooth scroll is enabled. The problem is worse when smooth scrolling is enabled, but is bad enough even when it is not.
Other applications seem to have scrolling slowed somewhat when Firefox has many windows open. It looks to me like a memory swapping issue, but I have no evidence beyond my gut. Right now Firefox has 30 tabs open on all its windows. Performance is bad, though not intolerable.