I use the "View First Unread" button, in each thread pretty often. I have noticed sometimes when i use the link it either take me a post i have already read, sometimes it will take me to a post that is above one of my posts. Other times it will just take me to the first post in the thread.
It seems to be an intermittent thing as sometimes it works fine.
I am using Firefox In Windows 7 & Mac OS X 10.6 - It does it in both.