Is this using the Yahoo mail website or some other program? If it's the website, then it would have to either be a browser configuration issue or a server side issue on Yahoo's end.
I'd be more inclined to think that this is a browser configuration issue. Reset the default configuration options on Safari, since every now and then it's settings can affect other programs even though it's not supposed to. Try it again after that.
I have a new iMac with Snow Leopard and do not have the following problem with my MBP using Leopard nor have I been able to duplicate this problem on new iMacs at the Apple Store.
Normally on other computers, the Yahoo Mail email header is at the top of the screen, the view pane is at the bottom of the screen and when I double click the email header, the entire email fills the screen in a new tab. On this new iMac, double clicking the header does nothing; the email stays split on the screen with the header at the top and view pane at the bottom and no new tab.
I've tried Safari, Firefox, and Chrome and the problem is unique to this iMac. Any suggestions? Is there a preference that I'm missing?