Can't compose an email in FF then opened IE and composed with no problems.

I wrote a note in MSWord and was going to copy and then paste it into a Yahoo email. When I opened Fire Fox and went to Yahoo Mail, I couldn't "Compose" a letter. Everything else worked fine but when I hit the Compose button, nothing happened. The "pointing hand" was there and the button blinked but it never brought up a blank email form. I then opened Internet Explorer and went through the same procedure and when I hit "Compose", a blank email form came up with no problem. This happened about 2 weeks ago as well and lasted about 2 days before I was able to compose an email using Yahoo Mail in Fire Fox.