This is the most feature-rich and flexible add-on for infinite scrolling. It handles tons of popular sites "out of the box", but most importantly, it allows very flexible custom setup for sites that are not handled automatically. And unlike some other extensions, Infy allows customizations via not only XPaths, but also CSS selectors and keywords. It may sound daunting at first, but A) it's not very complicated, and B) I don't usually need to worry about these things, since for most pages, everything just works.
Thank you for this fantastic add-on!