That’s by design.
You can do whatever you want with the dynamic identifier obtained this way, can’t you?? Just different from Chrome. (Different is good, because Chrome is terrible for privacy.) Tell the external service the identifier each time, but this doesn’t seem good for privacy. You need to be clear in your add-on’s privacy policy about its behavior.
The identifiers need to be different for each installation in order to make privacy-invading fingerprinting harder. But this should be done to improve the situation:
https://discourse.mozilla-community.org/t/proposal-isolated-content-environment/9866?u=desktopd