If you or they have upgraded to the "new" yahoo mail it doesn't work with Firefox. Had the same problem. Google and you will see all the people who are having the same problem.
I went back, and per one of the forum members changed my browser from the tools bar to trick yahoo into believing I didn't have the appropriate browser for the "new" yahoo mail, clicked no to upgrade and I went back to the classic yahoo that works with firefox perfectly.
Which BTW thanks to the forum member for that tip, it was priceless!
Since the new new yahoo mail allows and admits to scanning of everyone's mail and reporting it to the gov......I am going to stick with the classic yahoo mail as looooong as I can!