Claims 7-10% performance improvement on an old AMD FX thing. No information about the baseline though; whether it's the terrible Snap or Firefox's official binary package. I suspect it's the former because it has known performance issues IIRC and the latter has quite good compiler optimisations already (LTO+PGO making most of the difference).
When I built Firefox for x86_64-v3, I saw no measurable improvement over x86_64-v1 in speedometer. I didn't dare to build the most security critical application on my system with unsafe compiler optimisations though..