The extension itself does not collect or store any private information. However, if you wish to "Check for available public annotations" in the popup menu, every URL will be sent to the Hypothes.is server. While the request to the server contains only the URL, I assume that the server gets also the IP address of the machine making the request. Note that sometimes the URL address may also contain sensitive information. You can easily activate and deactivate this feature from the popup menu. On default, it is deactivated.