I'm using Firefox 2.0.06 on my Linux machine - I don't think its got enough memory for anything more. I'll nip back and try FF3 on my Windoze box.....
How wierd. The login box kept giving the error message, but then I logged in using the login area at the top of the forum page and it worked.
I suspect it may be something to do with the way the site tries to set cookies - Firefox 3 has much greater security than Firefox 2