When an extension is uninstalled, we should also clear storage.sync data stored locally, (similarly to how we clear IndexedDB, localStorage and storage.local data) but ExtensionAddonObserver.onUninstalled doesn't currently cover storage.sync.

I'm not sure we actually want to "fix" this. storage.sync is supposed to behave the same locally, regardless if user is signed into Sync or not.

Fixing this would create a difference, as those who are logged in will get back their storage.sync data upon reinstallation of the extension.

(In reply to :Tomislav Jovanovic :zombie from comment #1)

I'm not sure we actually want to "fix" this. storage.sync is supposed to behave the same locally, regardless if user is signed into Sync or not.

Fixing this would create a difference, as those who are logged in will get back their storage.sync data upon reinstallation of the extension.

That may actually be the reason why storage.sync data stored locally isn't currently covered by ExtensionAddonObserver.onUninstalled, but it still may be good to double-check the bugzilla comments from the bugzilla issues that have originally introduced the feature to see if we find any explicit reference to this rationale.

Related to that (storage.sync data upon reinstallation), we do also have another bugzilla issue that I haven't been able to come back yet (Bug 1511574). I'm adding it as a see also on this issue in the meantime because it may be also relevant.

I also think, the current behaviour is the better choice. It should not get "fixed" but the current behaviour should be announced officially.

The new Rust backend works the same way—if you uninstall an extension then reinstall it, all its settings will still be in storage-sync2.sqlite, so it'll have the same settings as before you uninstalled. Noting because Victor and team noticed it while testing, and he was curious if that was intentional.