Many of the comments on WebExtensions miss an important point: basing WebExtensions on the Chrome extension API does not imply limiting WebExtensions to the Chrome API. For example, Bill already made it clear that we want to support addons like Tree Style Tabs and NoScript, which Chrome does not support. So Firefox addons will continue to be able to do things you can't do in Chrome (though there will be some things you can hack into Firefox's XUL today that won't be supported by WebExtensions, for sure).
WebExtensions is something we need to do and should have done many years ago, before Chrome even existed. It's what Jetpack should have been. But better late than never!