Maybe it's geo-located? Because the Irish version definitely doesn't use Flash. Possibly the US version does...
It must be, because the link says
US.
It's terrible Most of the time when clicking on saved folders, it won't display emails, page refresh doesn't work. Clicking on the Yahoo Toolbar "Mail" will then allow folder contents display for a while. This occurs in IE8 & FF on different computers. It was beta for a while until it went final a month ago, now no choice.
This is the worst Yahoo email "upgrade" ever.