Note added to 114 beta with this wording:
WebTransport is now enabled by default and will be going to release with 114. As the original Explainer notes, it enables multiple use-cases that are hard or impossible to handle without it, especially for Gaming and live streaming. It covers cases that are problematic for alternative mechanisms, such as WebSockets.
Built on top of HTTP3 (HTTP2 support will be coming later). The current implementation in Firefox is passing 505 out of 565 Web-Platform Tests.