I'm using Firefox 1.0.7 on WinXP and I can tab to the link header, then hitting the ENTER key zips or unzips the menu below. Ice_Man, are you trying to implement their DHTML menu system on a site you're building? If you are we need the HTML, JavaScript and CSS used. A link to an example page would work just fine.
Although I don't know how the implementation of the DHTML menu, at this point, involves Web Accessibility. What problems are you having?