Created attachment 923610 [details, diff] the patch Related upstream issue https://bugzilla.mozilla.org/show_bug.cgi?id=1667664
Looks to me as if you'd rather want to make that be runtime configurable, not as a costly build time option.
The power of Gentoo is that you can easily add such patches locally in https://wiki.gentoo.org/wiki//etc/portage/patches. I don't see why we'd want to add this downstream if not merged upstream, especially not with a USE (we avoid conditional patching). But also, what asturm said.
