Internet Explorer 6 - 9 (we give a warning for people still using IE6)
Firefox 3 - 7
We support Chrome and Safari insofar that we fix bugs that are being reported, but we do not officially test them.
I believe that from up to IE 7 it is officially supported, however where able IE 6 is also supported. Still i would really really stress to convince your customer not to use IE 6 as it is not only out of date it is also a security hazard as Microsoft has stopped supporting it.