MetaMask Flask by MetaMask Labs

THIS IS THE CANARY DISTRIBUTION OF THE METAMASK EXTENSION, INTENDED FOR DEVELOPERS.

MetaMask Flask is a distribution channel of MetaMask Extension for developers, which gives them access to additional unstable APIs. The goal of Flask is to maximize developer control, so that we can learn the full extent of what developers want to do with MetaMask, and later incorporate these lessons into the main MetaMask distribution.

You can find the main / production version of MetaMask here: https://addons.mozilla.org/en-US/firefox/addon/ether-metamask/

MetaMask is an extension for accessing Ethereum enabled distributed applications, or dapps in your browser! The extension injects the Ethereum web3 API into every website’s Javascript context, so that dapps can read from the blockchain.

MetaMask also lets the user create and manage their own identities—via private keys, local client wallet, and hardware wallets like Trezor™. When a dapp wants to perform a transaction and write to the blockchain, the user gets a secure interface to review the transaction, before approving or rejecting it.

Because it adds functionality to the normal browser context, MetaMask requires the permission to read and write to any webpage. You can always “view the source” of MetaMask the way you do any Firefox extension, or view the source code on GitHub:
https://github.com/MetaMask/metamask-extension

MetaMask enables access to:

Web3
Dapps
NFTs
ERC20
tokens
ICOs
ERC271

...and more!