Hi Justin,
I've tried to reproduce the problem, but unfortunately I couldn't. Here are my steps:
1) Installed Windows XP Professional Version 2002 SP3
2) Installed Mozilla Firefox 21.0 and set it as default browser.
3) Tried to open
http://demos.telerik.com/silverlight/ - a dialog was opened with suggestion to install Silverlight 5.
4) I've installed the suggested version - Silverlight 5.1.20125.0.
5) Restarted Firefox.
6) Opened
http://demos.telerik.com/silverlight/ - everything worked as expected.
I've also tried to open
http://demos.telerik.com/silverlight/ in IE6 and it worked fine (it looks like Silverlight 5 is partially supported, but not recommended to use in IE6).
May I ask you to give more information about your environment. Also could you try reinstalling Silverlight 5 and updating Firefox to the latest version. Also you can try starting Firefox with disabled add-ons (from Firefox go to Help menu and select "Restart with Add-ons Disabled").
As for your question about Silverlight 4 - we are no longer distributing our controls for Silverlight 4, you can check our roadmap
here. Also
RadPivotGrid is not supported for Silverlight 4.
Regards,
Rosen Vladimirov
Telerik
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