Created attachment 470448 [details] /var/tmp/portage/www-client/firefox-53.0/temp/build.log firefox-53.0.ebuild from mozilla overlay fails during build
Try upgrading app-text/hunspell.
Reporter: Hunspell >=1.5 is needed. https://gitweb.gentoo.org/proj/mozilla.git/commit/?id=1d7283dd1feaffc903133d813ce7c82e3f7ed497