Extended Menu with Suckerfish template not working in Firefox

I am migrating a couple of sites from Mambo to Joomla and everything has been going OK. I replaced the JSCook menu with Extended menu for XHTML validation purposes. The implementation works fine on IE but for some reason the menu is not working correctly in Friefox.

You can see if you view this site in IE and in Firefox

http://www.cell-separation.co.uk

(the site is a work in progress and is not finished, the other sites replicates the exact same problem)

I spoke to the guy who made the extension and he said it could be to do with the size of the containing <div> but this looks OK, so I cannot see what is wrong. Both sites are due to go live soon and this is fast becoming a show stopper...

All help appreciated...