Firefox has begun supporting HTTP/3 in nightly builds. Firefox nightly is an official experimental channel that provides binaries to allow you to test new features such as HTTP/3. Nightly builds may have some stability issues that you should keep in mind when testing.
HTTP/3 is not enabled by default. It can be enabled by toggling network.http.http3.enabled value to true in about:config,
Launch browser advanced preferences by entering about:config in the URL section. Search the preference network.http.http3.enabled and make it to true.