The reason for the limited features in latest versions is due to how Firefox itself has been re-architected. Firefox has removed a number of key API that this extension originally used as they do not plan on re-adding them unfortunately.

For more information see:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Comparison_with_XUL_XPCOM_extensions#Privileged_APIs