extISessionStorage Interface

 Gets the value of a storage item with the given name. Returns a
 default value if the item does not exist.
 @param   aName
          The name of an item
 @param   aDefaultValue
          The value to return if no item exists with the given name
 @returns value of the item or the given default value if no item
          exists with the given name.

nsIVariant get(in AString aName, in nsIVariant aDefaultValue)

 Determines if a storage item exists with the given name.
 @param   aName
          The name of an item
 @returns true if an item exists with the given name,
          false otherwise.

boolean has(in AString aName)

 Sets the value of a storage item with the given name.
 @param   aName
          The name of an item
 @param   aValue
          The value to assign to the item

void set(in AString aName, in nsIVariant aValue)