AutoPager is a Firefox extension which automatically loads the next page of a site inline when you reach the end of the current page for infinite scrolling of content. There are several presets built into AutoPager, you just have to decide which to turn on. I'm primarily using it with Google and on a stock image site. This is one of the recommended addons for Firefox users which can be downloaded from here.

Besides the presets, the plugin will prompt you on other sites to create a new preset for that site. In fact that's what I did for the stock image site. I don't recommend turning this feature on for every site though. Keep in mind how resource intensive some sites are when loading pages and decide accordingly. Auto pager pre-caches pages and shows them in a single page so it is continually loading.