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Web dev: What browser is visiting the page?
User agent string:
A screenshot of a browser. The URL bar reads firefox://settings, a button on the URL bar is labelled Netscape, a popup from the button reads: "You're viewing a secure Opera page", and the web page title reads "Chrome settings".
So you don't use Firefox, you mess with Firefox. That's on you then. Devs can't be held responsible for you intentionally breaking things. Only do what you know works.