What is it?
Web Extensions allow developers to build features on top of browsers without requiring them to be implemented natively, in the browser.
When can I use it?
CSS Custom Properties is now supported in Firefox, and it will be available in WebKit soon. For a view of current and upcoming browser support, check out caniuse.com.
Learn how to enable this feature in popular browsers.
Where can I learn more?
You can learn more about the Extensible Web Community Group on their W3C website.