Are you suggesting that 10% is a reasonable estimate of the percentage of Firefox instances running on hardware that supports AVX? I'm not sure of the real numbers but this strikes me as an extremely low estimate.
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?