Fix info about the Browser Extension 'Testing Playground' (#787)

* Fix info about the Browser Extension 'Testing Playground'

* Fix FF-Link

Author: wuarmin

docs/queries/about.mdx

@@ -398,14 +398,13 @@ const { container } = render(<MyComponent />)
 const foo = container.querySelector('[data-foo="bar"]')
 ```
 
-## Chrome extension
+## Browser extension
 
 Do you still have problems knowing how to use Testing Library queries?
 
-There is a very cool Chrome extension named
-[Testing Playground](https://chrome.google.com/webstore/detail/testing-playground/hejbmebodbijjdhflfknehhcgaklhano/related),
+There is a very cool Browser extension for [Chrome](https://chrome.google.com/webstore/detail/testing-playground/hejbmebodbijjdhflfknehhcgaklhano/related) and [Firefox](https://addons.mozilla.org/en/firefox/addon/testing-playground/) named Testing Playground,
 and it helps you find the best queries to select elements. It allows you to
-inspect the element hierarchies in the Chrome Developer Tools, and provides you
+inspect the element hierarchies in the Browser's Developer Tools, and provides you
 with suggestions on how to select them, while encouraging good testing
 practices.