Firefox formatting of product page

Hi, I am using Opencart Version 1.5.1.3
It's a clean install (and my first opencart project!)

I have found that the formatting on Firefox (11) is not correct- the tabs on the product pages are offset, it's not unreadable but it doesn't look nice. The site works OK in Chrome, IE & Safari.

You can see the store here: www.buy-trees.co.uk and a screenshot of the offending layout here: http://screencast.com/t/PySDtVUdd

When I switch to default theme I get the same issue so I'm guessing it's something I've done somewhere else. I can't think where.

Can anyone offer any advice?

Thanks!
Amy