* Changes to Firefox's internal code should be less likely to break add-ons.
* WebExtensions should be easier to use than the existing Firefox XPCOM/XUL APIs.
* WebExtensions should maintain acceptable security and privacy standards.
Much of the specifics of the new API are similar to the Blink extension API. Google has [https://developer.chrome.com/extensions extensive documentation on the API]. [https://dev.opera.com/extensions/ So does Opera].