http://www.expertbettingadvice.co.uk/ I changed the top navigation menu on the page to add in a new link, fine in IE but Firefox doesn't like it for some reason. I added tipster competition and changed the padding in the style sheet for the buttons to padding:10px 20px 2px 5px; which in IE made the menu smaller to accommodate.
I just viewed your website in Firefox 2 and Internet Explorer 7 and I don't see a single problem with it. The navigation looks fine in both browsers for me.
Thanks Adam, that's strange, in Firefox it displays with the end two buttons stacked below the start for me.