Beware that there have been some issues with Google sites not loading or being very slow with Firefox's HTTP/3 implementation. Some of these were Firefox bugs and some were Google server bugs.
But I use HTTP/3 in Firefox Nightly and use Gmail and Google Docs all day long without major breakage these days.
But I use HTTP/3 in Firefox Nightly and use Gmail and Google Docs all day long without major breakage these days.