Smart Referer does not collect or transmit any personally identifiable information about you in any form and will make it harder for third-party content providers to collect data about you as well.

Please note however that the extension will by default connect every 24 hours and on browser startup to GitHub's pages service to retrieve to most up-to-date whitelist. GitHub will not process this data, except for gathering general traffic statistics (see their Privacy Statement) but would be technically able to infer that you are using this extension because of this. You can disable this behaviour at any time by opening the extension preferences (right-click the icon and select Preferences…) and unchecking the Use default whitelist option.