777: It may be running just fine, but does it
look fine to you? If you are using the current (stable) version of Yahoo! Mail, you'll see the navigation (prev, next, back to messages etc) on the top and bottom of each message you're reading.
Make sure you have the default stylesheets (no user sheets) defined in Opera... try a brand new install, and take a look at that bar. This is what I see:
Do you see the same?