Changes

We're using JAR signing for WebExtensions. , and Chrome uses a different system. Since signing is necessarily browser-specific, there's no point in integrating the two. Otherwisewould be difficult, and maintaining the current packaging extension for Firefox makes sense. The only part of the XPI file format that WebExtensions uses is the filename suffix. We haven't decided yet whether , and there may be some changes to the final extension we use .xpi or .zipas we seek (and get) more feedback.