Before using this add-on, you have to tell it what Mastodon instance you use.

It will ONLY ever communicate with that server, and ONLY when you interact with it.

The only information sent is part of the URL of the page you're on: as an example, if you use the add-on on https://EXAMPLE.COM/@SOME-PROFILE it -- assuming you are on a Mastodon instance -- takes that URL and finds the domain and username (@SOME-PROFILE@EXAMPLE.COM), then opens a page allowing you to follow that account.

It never accesses or modifies page contents, it never makes changes to your Mastodon account, and it never makes requests without actively used.