There was a warning on Yahoo a while back about an issue with Firefox and Flash, I believe. Just as you went to Yahoo mail this warning popped up saying you had Firefox and Flash and you may have trouble with Yahoo mail.
Let's update the software(s) in question and that should clear things up.
First, update Firefox by going to "Help" at the top right of the menu bar. You should see something about updating there. Click it and follow the prompts.
Secondly, make sure your Flash is completely updated. Go to the Adobe Flash website (Click here...
) and install the latest and greatest version. Make sure you're in Firefox when you do this!
If all else fails, use Internet Explorer or some other web browser to access Yahoo mail until everything shakes out. You'll probably get updates over time that will help this problem (if we don't fix it first).
Good luck! Remember to rate the solutions!