Just installed 13.04 with built-in Firefox 20.0. Strangely enough in Yahoo Mail I cannot view contents of any mail folder except Inbox. Any other folder (Sent, Trash, etc) when clicked won't open but for a fraction of a second, then disappear, and contents of Inbox are shown again. And it keeps checking for new mail every 5 seconds!
3.8.0-19-generic #29-Ubuntu SMP Wed Apr 17 18:16:28 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux