Unfortunately current AVD will stop working in Firefox 57 which is now in the beta and will be released on November 14, 2017.
Current AVD requires functions which Mozilla developers will remove since Firefox 57.
Our developers already building an entirely new AVD application from scratch in order to support Firefox 57 and another popular browsers.
Despite some difficulties with migrating old AVD code its main functional related to downloading process is already finished.
If new AVD will not be fully finished in time to the Firefox 57 release it will be published without some features which will be added later.
Current AVD version should still works fine in all Firefox versions below 57 despite marking by Mozilla as LEGASY add-on.