First, go to your add-ons in Firefox (it's under the menu "Tools", then "Add-ons") and there, you can disable or remove "Firefox Better Web with Scroll".
Next, go to Billing and "cancel membership". This will make sure your card won't be charged.
Then, click to Privacy in your settings and click "log me out everywhere". This should remove Scroll from all browsers, but you may have to restart your browser to see this change. (If you want to fully delete your account, you can also do this from the Privacy settings.)
For more about Scroll, please check out our homepage.