u2f-useragent-adapter is a browser extension for Firefox browsers that spoofs the user agent for certain websites that restrict the use of hardware security keys, based on which browser is being used.
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 (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.
The add-on can be installed from the Firefox add-on page.
This code is licensed as CC0.