This extension has the option to use Google Fonts to get more fonts. Their privacy policy can be viewed here: https://www.google.com/policies/privacy
Privacy information related to the font service can be found here: https://developers.google.com/fonts/faq#what_does_using_the_google_fonts_api_mean_for_the_privacy_of_my_users

"Use of Google Fonts is unauthenticated. No cookies are sent by website visitors to the Google Fonts API. Requests to the Google Fonts API are made to resource-specific domains, such as fonts.googleapis.com or fonts.gstatic.com, so that your requests for fonts are separate from and do not contain any credentials you send to google.com while using other Google services that are authenticated, such as Gmail."