If you want to scroll through your Facebook news feed (be it on the main feed, your personal feed, or in groups) to older parts, you have to scroll to the bottom, wait for the ajax load of the next part of the feed, and repeat. The problem with this is that, if you're scrolling down very far, the HTML document just gets bigger and bigger until your browser starts to die due to the overload of resources brought on by added HTML, text, and even images. This pretty much sets a limit to how far back you can scroll. Clicking on months and years on your personal feed has the same effect of cumulatively adding feed segments to the HTML document. I notice that this month/year feature is not available on the main feed and for groups.
If there were a way to literally page through the feed so that only a single page's worth of feed data is loaded at a time, that would make scrolling through it much more doable.