Are you perhaps thinking of the currently unused "class-A data"?
https://blog.mozilla.org/services/2014/04/30/firefox-syncs-n...
https://blog.mozilla.org/services/2014/02/07/a-better-firefo...
https://github.com/mozilla/fxa-auth-server/wiki/onepw-protoc...
Edit:
From what I can see, client side encryption in Google Chrome appears to be opt-in?