What browser do you use?
Do you have ANY addon installed? Is NoScript one of them?
If that's the case, make sure that yahoo.com and dropbox.com scripts are ALLOWED.
To write a new message, use the Write link at the left of the actions band, just under Yahoo! logo.
To reply to a message, use the curvy left arrow that is visible when you read any message.
The double curvy left arrow let you reply to the sender and all other recipients.
Electro, May 2013
