If you are working on transitioning your add-on to use the WebExtensions API, you have until about mid-October (a month before Firefox 57 lands to allow time for testing and migrating), to port your legacy user data using an Embedded WebExtension.
This is an important step in giving your users a smooth transition because they can retain their custom settings and preferences when they update to your WebExtensions version. After Firefox 57 reaches the release channel on November 13, you will no longer be able to port your legacy data.
If you release your WebExtensions version after the release of Firefox 57, your add-on will be enabled again for your users and they will still keep their settings if you port the data beforehand. This is because WebExtensions APIs cannot read legacy user settings, and legacy add-ons are disabled in Firefox 57. In other words, even if your WebExtensions version won’t be ready until after Firefox 57, you should still publish an Embedded WebExtension before Firefox 57 in order to retain user data.
When updating to your new version, we encourage you to adopt these best practices to ensure a smooth transition for your users.