The author is trying to figure out why GCC underperforms Clang for Firefox builds. Seems inappropriate to worry about cutting-edge features that probably are available to 10% or so of computers that run Firefox (or a competing browser.)
The closest I can find is Valve's hardware survey, which says that 87% of users are running on computers that support AVX. https://store.steampowered.com/hwsurvey (click on "Other Settings" on the bottom to expand).
Firefox is presumably lower than this, although I don't know by how much. Does Mozilla collect statistics on the capabilities of the computers that Firefox is being run on?
> Moreover GCC -O2 defaults are (in my opinion unfortunately) still not enabling vectorization and unrolling which may have noticeable effects on benchmarks.
... which makes it sound like Clang does a better job at this with Firefox code.