@ossobuco,
It's probably yahoo, as their "NEW" yahoo mail is very unstable.
To go back to YAHOO! MAIL CLASSIC turn off off the Javascript in your browser. Then try going to yahoo ans signing in, which then, you'll be asked if you want to return to Mail Classic, click "yes".
Since you have mac, this is to disable Javascript> Open SETTINGS>click PREFERENCES>>click SECURITY>>Uncheck ENABLE JAVA and ENABLE JAVASCRIPT
Now log out (just to make sure you setting is saved) and if you want you can close the browser and delete the cookies.
Then start up safari again and turn on JAVASCRIPT (with the same process above) and go to Yahoo mail, and see if this problem arises.
If the problem still exists, it's likely a glitch from Yahoo, so shoot them an email stating you problem.