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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
This is actually something completely different (an extension called PreMiD which basically TLDR; has a store full of "Presences" that users can just add in to their extension and toggle etc). It's extremely customisable and easy to use as well as support hundreds of websites and people can add many websites as well. The extension you're talking about is most likely completely unrelated, you can find PreMiD Presences @ https://premid.app/store and source code is on https://github.com/PreMiD/Presences/