why? Web Serial is one of the better web-based integrations out there. There is no functionality available without an explicit authorization, all that website can do is to pop up a dialog. And you approve individual port on per-site basis, so it's not like it can grab a random port without your knowledge.
This is my favorite model for web: anything more complex should be gated behind user permission. A site which requires that functionality will get that permission with no problem, and a random clickbait fingerprinter will get nothing.
As others have said, you'll get a dialog if any site actually tries to use this. But if you want to fully turn it off, just set "dom.webserial.enabled" to false in about:config (and restart Firefox)
This is my favorite model for web: anything more complex should be gated behind user permission. A site which requires that functionality will get that permission with no problem, and a random clickbait fingerprinter will get nothing.