So .. this is odd.
I'm on Firefox 72 (Beta), 64-bit running the the most recent release of Windows 10.
I came onto the forum like usual, opening up the main forum page like always through my usual bookmark (link:
https://www.thefurryforum.com/forums/index.php?action=forum).
I went to view my unread posts (clicking the "You have X unread posts since last visit.") and then clicked "Mark all messages as read" after reviewing the list of unread posts.
I noted that it took a little longer than normal to load and once it did I was met with the following page (attachment), seemingly logged into someone else's account. Now, browsing anywhere else (by opening a new tab) put me back to my own account and trying to click "Mark all messages as read" again gave me a "Session verification failed." error, so it doesn't seem very exploitable, at least.
No idea what might've caused it though, it just seemed to happen all randomly.