Firefox 90 will support Fetch Metadata Request Headers which allows web applications to protect themselves and their users against various cross-origin threats like CSRF and XS-Leaks. The fundamental security problem underlying cross-site attacks is that the web in its open nature does not allow web application servers to easily distinguish between requests originating from its own application or originating from a malicious (cross-site) application, potentially opened in a different browser tab. In total there are four different Sec-Fetch-* headers: Dest, Mode, Site and User.”]
Source: https://blog.mozilla.org/security/2021/07/12/firefox-90-supports-fetch-metadata-request-headers/