Since last Wednesday (Sept 12) new posts on my Facebook Newsfeed have been disappearing. I used to be able to scroll down into the past forever, but now I am lucky to see five or more posts at a time before getting a "Do You Want To See More Posts?" notice at the bottom, a notice I used to never see. Sometimes a new post will appear and then vanish if I leave the page and come back. There are a small few posts that seem to stick longer than others, but they don't last forever. All the newsfeeds I have subscribed to are still listed in preferences.
The exact same situation happened to me in the middle of last month, then seemed to go away after a few days, though I don't think it ever showed earlier posts and just let them accumulate again.
I'm using Firefox Quantum 62 and have Avast Antivirus.