You can already do this. In Firefox: * Go to your Cookies And Site Data preferences * Select "Block Cookies And Site Data" * Click on "Exceptions" * Add th

You can already do this. In Firefox: * Go to your Cookies And Site Data preferences * Select "Block Cookies And Site Data" * Click on "Exceptions" * Add the address of the website you opt-into * Click "Allow For Session" or "Allow" as you choose.