Mozilla Firefox 5.0 may not be official yet. However, you can still get hold of them now, if you want. It seems that Mozilla has uploaded them to their server in Linux, Mac and Windows flavors. So, if you want to try something new, try Firefox 5.0 now (I will wait for the official one).
So, what are the changes? According to Engadget, it will include support for CSS animation and a more easily accessible do-not-track setting. I will wait for the official (on 21st June if everything is ok).
BTW, on the FTP folders, it also shows a HTML file that indicate that the team has still not yet finish qualifying Firefox 5.0.