Firefox 54: E10S-Multi, WebExtension APIs, CSS clip-path

The release of Firefox 54 completes the transformation of Firefox into a fully multi-process browser, running many simultaneous content processes in addition to a UI process and, on Windows, a special GPU process. This design makes it easier to utilize all of the cores available on modern processors and, in the future, to securely sandbox web content. This release also offers new support for the CSS clip-path property, and updates to the WebExtensions APIs.