Keef,
I've observed that happening on a number of threads. It does seem to be a problem with Firefox. Try adding ?v=nocache to the end of the URL and reloading. If you can see the missing posts then it's the same problem.
You will also be able to see later posts by replying to the thread and scrolling down, without actually replying. Also, if you log out and view as a guest you should see all the posts.
Do you have NoScript running?
We are aware of problems on threads where a lot of posts have been deleted and / or a lot of thread bans have been implemented.
SD