I have the same problem with Firefox and Chrome, but not IE 8.  I e-mailed Yahoo about it, but they passed the buck to AT&T support.  Rather than waste my time with AT&T I figured I'd check here to see if anyone has a solution.  I upgraded to the latest version of Flash, but no soap.  My guess is that it's Yahoo's problem, but they're pretending it isn't so we'll bug AT&T about it.

Here's the full text of the message, in case it means anything to anybody:

"Flash required for this application

Your browser is blocking Flash. You must have Flash 9.0 or higher installed and unblocked to run an openmail application. You can download and install the latest Flash Player here "

Flash works fine on every site I've tried, and the webmail works fine after I dismiss the error message.   One workaround is to switch back to Mail Classic, if you don't mind that it doesn't open the inbox by default.

Googling finds lots of people complaining about this on various forums in the last few days, so it's probably Yahoo's problem rather than AT&T's.

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oobleck