CSS Grid Layout is a game-changer for web developers. Learn more about CSS Grid and the CSS Grid Layout panel in Firefox.

Learn by example: cssreference.io is a free visual guide to CSS. It features the most popular properties, and explains them with illustrated and animated examples.

When the code has stopped at a breakpoint, you can examine its state in the variables pane of the debugger:

WebRTC (where RTC stands for Real-Time Communications) is a technology that enables audio/video streaming and data sharing between browser clients (peers). As a set of standards, WebRTC provides any browser with the ability to share application data and perform teleconferencing peer to peer, without the need to install plug-ins or third-party software.

WebRTC components are accessed with JavaScript APIs: the Network Stream API, which represents an audio or video data stream, the PeerConnection API, which allows two or more users to communicate browser-to-browser, and the DataChannel API that enables communication of other types of data for real-time gaming, text chat, file transfer, and so forth.