Well, I asked him on Mastodon about IronFox not enabling this by default and this was the answer
“well, we currently don’t enable those by default due to us finding them to still unfortunately be too experimental/buggy in testing
but we’re keeping an eye on them and hope to enable them by default in the near future. for now, we at least allow users to enable them manually if desired from secret settings (and afaik we’re the first/maybe only? gecko browser to support enabling them at all, because we also enable support for it at build-time when building gecko/geckoview)
so you’re welcome to enable them, and if they work for you/don’t cause issues, you can keep them on (if you do decide to enable them, please lmk how your experience is! we def need feedback on it).”