Firefox is shipping W^X for jitcode, as far as I know. Or at least https://bugzilla.mozilla.org/show_bug.cgi?id=1215479 is marked as fixed and I don't see any obvious bugs blocking it that are fallout from that change.

And the "Firefox (Ion)" and "Firefox (non writable jitcode)" lines on https://arewefastyet.com/ seem to coincide...

But yes, actually making this work in practice is not at all simple.