Yahoo -- get with the program !
Silly buggers have been pestering users to change browser ever since they introduced a new look a few years back that wasn't compatible with my then choice, Opera. Eventually, fed up with being nagged I switched but frankly I'd rather they'd stuck with Yahoo's classic look or put in the effort to match most popular browsers.
I've largely switched to Firefox after a flirtation with Chrome, so the new nagging doesn't bother me. But Yahoo's performance in the past few years has been pretty poor (not to mention the recent underseas outage) and I suspect that this latest move will push some users out the door.