no,
for instance; some of the yahoo games and new yahoo mail photo (the one using for adding photos to mails easily) does not work in firefox.
the yahoo toolbar works fine though, but doesnt provide tabs like it does for IE (obviously cus firefox has tabs already)
It's fully functional in Firefox. I'm using Firefox right now with no problems whatsoever.
In fact, things work BETTER in Firefox than Explorer, since Firefox is able to handle the more advanced coding Yahoo has that Explorer can't handle.
If you're having troubles getting Firefox to play games or anything, though, be sure that you have all the plugins installed, as these are required to run certain programs (Like Yahoo! Games).