I think it happens because of the speed of your internet connection. Wait few minutes after entering the url. Then if the Yahoo mail beta don't open then it will show another link to login to normal version of Yahoo mail.
Thank you
I don't think yahoo Mail Beta has some problems with FF. There might be some issue with your screen's resolution but I haven't read any issue regarding FF.
I prefer the beta. It's stable and has several advantages over the current Firefox 2. The first full release version of Firefox 3 should be in a month or two, but the beta is fine for now.
You can also try this to avoid such kinds of problems.