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.
For end users still using IE6 you have the option to install Google Chrome Frame. It is free, you do not need administrator rights to install it (recent improvement) and it improves JavaScript performance and standards compliance by leaps and bounds.
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 Still i would really really stress to convince your customer not to use IE 6 as it is not only out of date it is also a security hazard as Microsoft has stopped supporting it.