If you face this error accessing SilverLight pages:
Error message: A Silverlight application error has occurred.
System.IO.IsolatedStorage.IsolatedStorageException: Operation not permitted.
then you should clear SilverLight isolated storage contents on your client machine under a user account you are logged in. Simply go to this folder and delete all files in it:
in Windows 7 / Windows 8 / Vista / Server 2008
in Windows Server 2003 / XP
C:\Documents and Settings\[user name]\Local Settings\Application Data\Microsoft\Silverlight\is
If the above folder is not visible you can enable showing it in the Control Panel - Folder Options - Show hidden files, folders, and drives.
Also, if after applying the mentioned above solution this issue still reproduces in Firefox, then most likely your Firefox is not set to save a history. In order to check and correct it:
1) Open Firefox;
2) Press ALT button to show up a menu;
3) Go to 'Tools' -> 'Options', click on 'Privacy' Tab;
4) In the 'History' drop-down choose 'Remember history' and click OK to save changes.