I don't see where to turn on cookies, but I think it is because I'm automatically logged in when I return.
The same old threads still show as having new posts, though more recent ones don't.
My problem is that the small circular icon isn't going away for several threads in Geddes' forum even after I've returned multiple times.
And What's New isn't a good solution, I don't want to have to sift through threads in forums I don't normally visit.