Since the Yahoo! Mail switched to the all new beta, I’m having trouble with Reply and Compose buttons not working. It only happens when I’m on my job, behind a proxy server, but still bugs me that I’m not able to send any email.
For some time now, there is no option to switch back to classic mail from the new beta, so I needed to find a new solution.
After a lot of searching I’ve found this links that point to various versions of Yahoo! Mail, and it helped me getting my mail to work. Here they are, hope they will help you if you’re having the same trouble.