Recently, Yahoo! changed something in their email protocol which made the Webmail Compose add-on for Firefox stop working. I've been able to get it to work again - at least on this computer - by modifying the following string (located in the Webmail Services tab in the add on)
Note: I haven't had a chance to test this on several computers yet, but my hunch is that the us.f329 part of the above string may need to be changed to the country-code/mail-farm you're logged in with (look in the address bar when you're in Yahoo! Mail to get it).
Hope this helps!