WebExtensions are the future
The current plan is that Firefox 57 will only support web extensions. Your add-on is not a web extension yet as far as I can see, so you should consider remaking your add-on. The Firefox 57 release is planned for 28th November 2017. Already for Firefox 53 (planned release: 18th April 2017) it is planned to no longer accept other new add-on submissions than web extensions on AMO (addons.mozilla.org). Old extensions can however still be updated. The good message is that one should be able to use your web extensions add-on in all mayor browsers. That's the idea behind web extensions. Note that you can use so-called hybrid extensions for switching to WebExtensions.
Note that all dates are estimates and may be changed.