Project Reality Forums

I use 'userCP' alot as I like to keep track of the threads I have contributed to.
When I have done reading/posting I click 'Mark forums read'

I come back to userCP a few hours later or next day, whatever and 1, 2 or even 10 random subscribed threads will be highlighted as 'not read' even though I know I have read them, sometimes my post will even be the last post on the particular thread(s).

I am using Firefox.