I have a profile that is failing to register new service worker scripts and is throwing an error when loading existing service worker scripts. Unfortunately its just NS_ERROR_FAILURE. Interestingly, though, these existing scripts still run even though an error was thrown. This reproduces across browser restarts. A fresh profile does not show this problem. I used sqlite to verify that the cache db schema is correct. I also removed the cache directory for the origin and it did not help. The serviceworkers.txt file looks correct. Starting a debug build of release to investigate.

Seems the profile has .metadata-v2 files in it. These are only produced in FF49+. So I accidentally updated the profile at some point.