Here are the details for the storage locations. Hopefully this helps you to narrow down which side of the connection is not working properly and we can take it from there.
(<FIREFOX_PROFILE_LOCATION> can be found by typing
about:profiles into your address bar)
The JSON data stored within that file may vary in future but at the moment the encryption key will be stored in a pair of properties like this:
NB: This file location is correct as of Firefox 59 but may be modified at Mozilla’s whim so if you can’t find it, double check with Mozilla where it should be for your version and operating system.
In an XML file located according to the documentation at https://keepass.info/help/base/configuration.html there will be some XML similar to this: