u2f-useragent-adapter by Firefox user 14699266

u2f hardware security keys, like the Yubikey, add a second layer of authentication when you log in on websites. The Firefox and Chrome browsers both support these security keys, but some websites like Facebook check the browser's user agent, and don't allow them to be used when logging in using browsers other than Chrome (they will instead ask for alternative second factor methods, like TOTP). By pretending to be using a Chrome browser during the login, it is possible to get around that restriction, and use the security key as usual.