But the functionality does not require a unique user id to be sent to the stats server.
Anyways, that is why it was made easy to shut off completely in case people didn't want to deal with it. Also making this behavior very clear in the next update within the extension itself (not just the welcome/upgrade pages). Privacy is critical.
Most people upgrade their plugin via Firefox's addon manager, and skip the "what's new" page that opens up. This is because we trust whoever made the plugin to not introduce any malicious behavior in future updates.
Judging by the intrusive nature of this feature, I believe it should be opt-in rather than opt-out.