A minority of web pages cause scrolling in Firefox to slow to a crawl. Touchpad and arrow key control is very slow indeed; scrollbar dragging is slightly better but still much worse than that would normally be.
It does not happen in any other browser.
Most of the problem pages are fairly long with quite large HTML files, but not always: so there is no clear causal link between them.
One problem site has a fixed background (which can cause this sort of problem for me, to a lesser extent), but hiding it with an entry in userContent.css hasn't improved things at all.
Sure. perishablepress.com is a good example. It has a couple of fixed elements, fairly long pages with quite large file sizes, but not the worst that I've seen. Yet for me, it scrolls like it's in treacle.
Spec is: Pentium Mobile 1.86GHz; 2GB DDR2; dedicated graphics of some kind but no clue what it may be; and a Synaptics touch-pad. All drivers are as up to date as I can get them. But as I mentioned, other browsers (Chrome, Safari, and Opera) don't show this problem at all.