This has been going on a while with MSN, only lately with Yahoo.
When I try to login to Yahoo, it just reloads the login screen. If I enter the wrong password, it tells me explicitly so, it only reloads when I enter it correctly.
With MSN, is seems to be retrying to access the login server, and failing.
I can log in fine in IE (technically, an IE tab in Firefox) to either service.
I tried clearing all cookies.
I tried clearing all cached data.
I tried disabling all extensions.
I even tried clearing all temp data on disk and restarting after clearing all cached data and cookies.
Using Vista Ultimate with all updates.