When I upgraded from Ubuntu Quantal 12.10 to Raring 13.04, my Yahoo mail in Firefox went bananas. It started checking for incoming mail every 1.5 seconds or so, making it impossible to go anywhere but the inbox or to answer any email.
The problem comes from the Yahoo mail webapp for Unity, and the fix is easy. Simply open Synaptic and uninstall the package unity-webapps-yahoomail. Or from the terminal, type:
sudo apt-get remove unity-webapps-yahoomail
After that, Yahoo mail will work fine in Firefox. Just ensure that you don’t enable the Yahoo mail webapp if asked again by Ubuntu.