Have you allowed Firefox to save the cookie, Gill? I know I keep on about the damned thing, but it has been the answer to a hell of a lot of queries in the past, especially of this nature, and of staying logged in.
I have found that Firefox will occasionally open two windows instead of one, and this leads to the 'disappearance' of the New Posts
icons. You can always check by going to the Home page, and click the Go button next to the drop down menu, by the view new posts, er, thingy. :|
I have to say, we have had a lot of users querying the New Posts systems on this forum. Personally, I don't find them a problem, and have got used to the way it works, but it's not the most user friendly feature of this forum software.