I've recently installed the vsphere web client 5.1.0 on my 2K12R2 box. I get the well known and documented issues when I try to connect using IE11 (but note that I can still connect to the vCenter server). So I decided to try firefox and chrome.
With Firefox I simply get an unable to connect error message - Firefox can't establish a connection to the serer at localhost:9443. When I try Chrome I get "this web page is not available" With IE it come up with the problem with the security certificate, but if I continue I can get in. The other browsers are set to use the system proxy settings so I don't' know what's going on.