Broken "New Posts" display

When I logged in this morning I got the first screen below. What I should see is a list of threads updated since I last looked (because my last action yesterday was to click "New Posts")

So I clicked on "New Posts", which produces the second screen, and then "here" gets the third screen, on which you can see that there were a lot of updated threads -- more than a screenful -- but with none of them in bold type (which they should be if I haven't seen them, which I haven't).

They have been updated since my last visit, and I have not marked them "read".

This used to work the way you'd expect, but now it doesn't. (The same problem has always happened for me with the "User CP/ View All Subscribed Threads" display -- it hardly ever shows the correct status.)

Is there anything I can do to fix this? Clearing cookies makes no difference.

[Using Firefox 3.0.11 with VistaHB.]