No, don't think so -- I don't play with anything along those lines. I'm thinking it's much more likely related to Netscape 4.73. (I did say "ancient". You thought I was kidding. Popup blocker? I settle for avoiding sites with popups when I'm on this machine.) ) I do have a Firefox on here, & don't see it with that, FWIW.
It has to be the most recent post in the forum to do it. So, if you are reading from the bottom up, and using the next thread option to navigate, when there is not a next thread to go to you get the mark forum read option.