Skipping pages when leaving a thread before I catch up.
Sorry for the strange title, can't really think of a more concise way to put it.
Anywho, sometimes I can't keep up with threads and get behind so I end up catching up a page or two at a time. In the past I'd just close the thread when I got to the bottom of a page and then was able to pick up again on the next page the next time I opened it. Recently, I've noticed that it has been skipping a page when I go back, so for example if a thread has 80 pages and I close out of it on page 60, having never read anything beyond that, the next time I open it I am on page 62.
Using Firefox Quantum 59.0.3 (64 bit) if it matters. I believe it has been happening at both work and home, I'm at work now so will have to check at home later.