MetaMask Legacy Web3 by danfinlay, rekmarks

The MetaMask extension provides an API to websites you visit so they can interact with the blockchain. In the past, we used to provide a complementary API known as window.web3. As of December 2020, we stopped injecting window.web3, which improves security and performance, but can break older and unmaintained websites. Using this extension in combination with the MetaMask wallet enables you to use those older sites as before.