Script Summary: Count User posts on a thread.
When installed, the script will change the post count to a link.
Click the Post Count, for the thread. The last page will load, when it is done counting... or close to done.
Open the Error Console in FireFox (CTRL+SHIFT+J)
Scroll to the bottom, and that last entry "GLB/User Count: PAGE X" will be the total posts in the thread, by person.