How to receive a real-time data to my extension

I want to create an extension that will show the stock market information for real-time. Is it any source or API are available to help me for coding?

To learn about how to create the extension, I suggest you begin here: https://developer.mozilla.org/en-US/Add-ons/WebExtensions

As for APIs, you’ll need to look into stock data APIs, which are probably abundant.