This issue is actually caused by Yahoo Music's site. The code on their site is doing something called "User Agent sniffing" (Browsers like Firefox, IE, and Google's Chrome, are called User Agents). What the code does is try to find what browser you are using and do something specific for that browser. The problem here is that now that Firefox has updated to version 4, Yahoo's code doesn't recognize the new User Agent signature, and looks to be defaulting to thinking Firefox is some version if Internet Explorer. It then tries to load a bunch of code intended for IE and that is what breaks the page.

Unfortunately, this is something only Yahoo can change (UA sniffing is generally bad practice for site to do for these exact types of issues btw) and they likely will when the realize it. I will reach out to the Yahoo team and let them know the issue.

In any event, I am pretty sure the site won't take long to make the necessary changes, they probably don't want to lose a bunch of users!

Thank you, dbuc! Truly appreciate your explanation as I was beginning to think I asked a twilight zoned question. In the meantime, I scrambled back to FF 3.6.12 just to get the videos playing again. However, FF4 DID play all other website videos just fine, if not better. You're the best!

FWIW I filed https://bugzilla.mozilla.org/show_bug.cgi?id=659634 long ago, hopefully Yahoo! would fix it one day =)

You're so on top of it, Alex! I hope they fix it, too.

Just trying to get a better web ;-) Can you please help sending them some feedback pointing them to the bug? Perhaps Yahoo! Music would then realize I'm not the only one using Firefox =(

Done.