Sync only works for Add-ons which are hosted at the Addons.Mozilla.Org (AMO) website. I doesn't actually synchronize each users own installed extensions, it installs them afresh from the AMO server. As far as the data for each individual addon / extension goes, my experience is that there are vast differences with what does and doesn't get synchronized. Some extensions have Sync-ability enabled by default, while other extensions have it hidden in about:config prefs which aren't always stated for users to avail them self of the hidden feature. That said, I haven't used Web Extensions in Quantum Firefox long enough (I am a late adopter) to see if there has been an improvement made over the last 10 months or so to evaluate the situation. Plus I have no need for Sync now-a-days with no Smart Phone or tablet, and no need for a Laptop since I retired; and I am not setup the "play with Sync" right now between my two desktop PC's on Quantum until more of the extensions that I use have comparable WE (WebExtension) versions to get into Quantum "whole hog". Bottom line is that Add-ons the user gets from anywhere other than AMO just won't synchronize thru Sync and your Firefox Account. Those extensions need to be installed from their source and the data needs to be copied over manually to additional devices; it may be possible to add "data sync" for those extensions to what gets sync'd thru Sync if the WE has a pref automatically enabled or has a "toggle switch" built-in or a selectable user preference in the extension interface. And that information would obtained from the developer of each WE involved. ''Based upon my observations and experiences with Sync and the Weave extension that was used to develop the Sync feature prior to it becoming a built-in feature in Firefox 4.0. Close to 10 years using Sync, and "playing with" Sync as an "interested party".''