Looks like you need to clear the cache and refresh the page. The css stylesheets are not loading. You can do that in Tools, Clear private data. Keep the passwords and forms if you store them, no need to clear that. Is the only thing I can guess right now :/, the code must be looking for a css file in the cache and it doesn't exists since it changed after yahoo renovation.
thank you for the responses
finally I found the problem
it was because the content filter (dans guardian) in clarkconnect server :-(
it blocked yimg.com
after I put that domain in white list, new yahoo mail classic displays correctly
but I still have no idea, why did the "problem" only happen on browsers in VL