Firefox globally gets passed the -mno-avx flag, which is not necessary. The problem is in just one component. Instead, https://hg.mozilla.org/mozilla-central/rev/3023f9390942 should be applied to the affected versions (see upstream bug https://bugzilla.mozilla.org/show_bug.cgi?id=680547). This way, only the affected component gets built without avx, while the rest of the browser can still benefit from it, on corresponding hardware. That patch will probably hit upstream releases soon, but in the meantime, Gentoo has a lot of older (and the current) versions that can benefit from it.
I have added 38.0_beta7 to mozilla overlay which no longer forces -mno-avx. At this point in time I am not interested in back porting the patch for prior releases. 38.0 will be the next esr release so it will be picked up rather quickly.
firefox-38 without the filter is in the tree now, closing this. Thanks!
