PROBLEM RESOLVED
I have a collapsible list on http://www.sissinghurst-hostas.com/directSales.html
at the bottom of the page. It works fine in IE6 and IE7, but does not work at all in Firefox.
OK, I got it to work, but I don't understand why it was an error. When I removed the </li> at then end of each item, the list works. Why?