Ralph asked: "OK, so am I actually making my browser think I want to go back to IE6 or am I just lying to Yahoo! about the version? I'm afraid I'll get into trouble if I pursue the fix suggested above. What does everybody else think?"
Ralph, if you follow the directions above, you will be asking your browser to lie to Yahoo! about which browser you are using. Moreover, the switch is temporary, as soon as you close the window or quit the browser (depending on which browser you are using, I'm told) it reverts to the ordinary browser you had. Moreover, the browser makes no attempt to behave like say, IE 6, it just tells the website that it is IE 6, so if you find an old cranky website that does need old and possibly buggy IE 6 behaviors, it won't really work in rendering the pages the way they used to be. But in this case it's useful to get Yahoo! to let you go back to the classic mail.
Reading these messages I'm glad I haven't jumped to the new Yahoo! mail -- after the last few times they "improved" their groups and mail, I've been just waiting until the very last possible moment to switch, not very happy with the changes, the older, simpler and cleaner versions they used to have used to be easier and faster. Oh, well.