You are asking about "ArcGIS Explorer Online," which is a depreciated online tool, that is still functioning but not supported anymore.
This is the forum for "ArcGIS Explorer for Desktop," which is a client based program, not a browser based program.
While there are no official statements about depreciation of ArcGIS Explorer for Desktop, it is not looking promising. The current release doesn't work on Windows 8.
Soon there will likely be no free viewer with ESRI.