On the homepage it admits that its not 100% libre: https://floorp.app/en/
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The source code for Floorp is mostly public, allowing anyone to view it and contribute to Floorp.
Usually I would put this under not accurately translated text from Japanese. But with your findings you might be on to something. Or they hide that part of the code because it was malicious or embarrassing and won’t be part of the program in the future? If this turns partial closed source, man, that would suck. What is it that so many closed source browsers exist?