Metamask-like web3 inject for bitcoin Is there a browser extension that works similarly to Ethereum's Metamask? I am wondering that if there isn't one, is it because of some technical limitation, or that just no one has gotten around to developing one.
There is a Bitcoin wallet extension for Chrome. It's apparently used by 1500 people and has 3.5 stars average review. Still, I would not trust this extension without first auditing its source code. I cannot find its code on its author's Github profile.

CoinSpace is a web-based Bitcoin wallet. It's not exactly an extension but if you understand what Metamask does you'll see that it isn't really different.

Metamask loads up an Ethereum javascript library and offers a front-end to it using HTML. This is exactly what other similar wallets do, e.g. myetherwallet. 

Brave Browser used to have a Metamask-like wallet for Bitcoin. I don't know what happened to the code when they moved to BAT tokens but it should be out there. It was basically a Bitcoin wallet in a Chrome extension, so the javascript for Bitcoin functions could probably be refactored out of the extension.