Back in 2015, Google introduced SPDY as a method of improving TCP connections. HTTP/3 now improves upon that by removing the blocking of TCP with the use of UDP (QUIC).
Firefox: currently disabled by default in version 85, to enable use about:config and set network.http.http3.enabled = true
IOS Safari 14+: currently disabled by default, but can be enabled under Settings > Safari > Advanced > Experimental Features > HTTP/3
Chrome/Chromium: current versions 88+ are currently implementing by default.
Chromium Edge: as new versions are based upon Chromium, support should follow Chrome.
MSIE: was never and will never be implemented.