Created attachment 428594 [details] build.log It does build if I set "SHELL=/bin/bash" in /etc/portage/package.env for firefox.
Created attachment 428596 [details] ebuild environment
Is firefox-45.0.1 from ::mozilla-overlay or from ::gentoo ?
From ::gentoo.
Fixed in gentoo repo (at least it works for me here, with /bin/sh -> dash). Will be committing to mozilla-overlay shortly to fix there as well.
Fixed for me too, thanks.