Each Substrate chain has its own address format. For example, an address starting with “5” is a generic Substrate address and an address starting with a capital letter is a Kusama address. 

If an account is used on all chains, when switching networks the address format the Polkadot-JS UI displays will change to that network's address format. That is the default behaviour. You can learn more about it here.

If Polkadot-JS UI is showing the wrong address format for the network you are on, here is how to change it: 

  1. Go to the Settings tab
  2. Select "Default for the connected node" for both "address prefix" and "default icon theme".
  3. Click Save
  4. Your addresses will now always show in the correct format, whether you have Polkadot or Kusama selected on the top left. 

Polkadot extension

In the Polkadot extension the address format depends on the chain you have chosen for that account. If you have chosen "Allow on any chain" the format will be the generic Substrate format, but in Polkadot-JS UI it will change when you switch networks, as discussed above. 

If you select a specific chain, then the address format will be of that chain and the account will be available only on that chain.