I noticed this recently: I open firefox to look at something. My homepage is the forums. The forums home page loads up. I look and see a few red cycles, and think "Ima read that later." I do something else, close firefox, then I reopen it later to read them, and the cycles aren't red and I can't see what I haven't read.
That's normal. If you close and reopen Firefox, you start a new session. phpbb assumes you read everything you had a chance to read in your last session so it doesn't have to store who read what for all thread/user combinations.
I made my google start page my home page, and then when I upgraded to firefox 3 beta5 (forced upgrade from Kubuntu), it started opening up all previous tabs. I like that feature so much, who needs a home page?