Very good structure, easy to follow (diagrams and screen dumps helps a lot :-).

The description for adding SENSEI properties in Firefox states:

extensions.sensei.accounting.symmetric.userKey
Enter the secret key you negotiated with your Framework Provider here.

But there is nowhere mentioned how to negotiate with "your Framework Provider". I have no idea what I should put here...

Aha, it came later, may be worth describing to set it to ade0db88dc614623498f5c53c2e1accb8cac84c594e478fe1b822555 in the first place?