I had an odd effect when I installed PDF creator tool bar on a windows 7 PC with both Firefox and MS Explorer. Opera still worked on this page and following uninstalling the toolbar, both Firefox and MS Explorer were also ok once more.
This is a simple private site the first page has areas that can not be shown until the user logs in and it may be this that give the problem although I have not seen any problem with this page and none of the users have had access issues. If you would like to take a look for yourself go to http://www.chrischambers.org.uk/bluebell_s_and_t/ to see the effect. It looks at if you are using Druple yourself so you may well have the answer to hand