I've been having this problem with Safari for weeks - when I logged in for the first time during the day, it wouldn't show me which threads had been recently updated - So i'd have to remember the last time I'd posted and then check which posted where new.
Now it won't remember my details at all . . . which means I can actually see which threads have been posted in, but I have to log in, each, and, every, fucking, time . . .