I noticed this about a month ago but finally got around to reporting.
Firefox version 120.0.1 (64-bit) (it's getting the latest version now, but the issue has happened with the prior version as well)
At first I thought it was possibly the ad blockers, so I've disabled those and duplicated.
ANY forum (doesn't matter which), if you go to Forum Tools- > Mark this forum read
you'll see the screen "shift" but all the "unread" threads still show up in bolded font.