How to copy folders between yahoo mail accounts?

I have just created a new yahoo email account and want to copy some folders from my current email account to the new account. I need to copy about 20 folders and 5000 emails. It would be extremely tedious to copy one email at the time from current account to the new account.

There must be better ways?

Would be grateful for advice.

Ken
P.S. using Firefox on Linux Ubuntu 12.04