I use HTTP/3 since a few releases ago. I had to set the "network.http.http3.enabled" to "True". No issue so far (tested on a high-quality FTTH home connection a

I use HTTP/3 since a few releases ago. I had to set the "network.http.http3.enabled" to "True". No issue so far (tested on a high-quality FTTH home connection and low-quality coax) with Google's website (including YouTube) and some websites under Cloudflare HTTP/3-ready CDN. I use my own fork of the "http-and-ip-indicator" extension to know when HTTP/3 is in use[1]. It shows an orange bolt when HTTP/3 is in use for the top-level document. [1]: https://github.com/enzzc/http-and-ip-indicator https://github.com/enzzc/http-and-ip-indicator (Beware, I don't maintain it actively)