Privacy policy:

This app stores data about tabs and windows that can contain personal data for example in the url or favicon-url, for the normal functioning of the app. Data is stored using a Firefox api in the local app storage of the browser and is never transmitted anywhere else by this app, this app isn't responsible for what Firefox does with the data once its stored or deleted, deleting the app should delete the data but since its storage is managed by Firefox that could change; by using or installing this app you acknowledge and give full permission to this.

Feel free to check this privacy policy for changes after an app update