Error: USE Flag 'clang' not in IUSE for www-client/firefox-60.2.2

[mrhnhrm]

Hi, and thanks for taking a look.

So, trying to merge www-client/firefox-60.2.2 here. I have seen some cryptic error messages from Portage, then I have seen some weird error messages, and now this one wins the cake:

[ebuild  N     ] www-client/firefox-60.2.2::net-kit  USE="dbus eme-free gmp-autoupdate hwaccel jack screenshot system-harfbuzz system-jpeg system-libevent system-libvpx -bindist -custom-cflags -custom-optimization -debug -geckodriver -hardened (-neon) -pulseaudio (-selinux) -startup-notification (-system-icu) (-system-sqlite) -test -wifi" L10N="-ach -af -an -ar -as -ast -az -bg -bn-BD -bn-IN -br -bs -ca -cak -cs -cy -da -de -dsb -el -en-GB -en-ZA -eo -es-AR -es-CL -es-ES -es-MX -et -eu -fa -ff -fi -fr -fy -ga -gd -gl -gn -gu -he -hi -hr -hsb -hu -hy -id -is -it -ja -ka -kab -kk -km -kn -ko -lij -lt -lv -mai -mk -ml -mr -ms -nb -nl -nn -or -pa -pl -pt-BR -pt-PT -rm -ro -ru -si -sk -sl -son -sq -sr -sv -ta -te -th -tr -uk -uz -vi -xh -zh-CN -zh-TW" 0 KiB

(...)

 * ERROR: www-client/firefox-60.2.2::net-kit failed (configure phase):
 *   USE Flag 'clang' not in IUSE for www-client/firefox-60.2.2
 * 
 * Call stack:
 *          ebuild.sh, line   92:  Called src_configure
 *        environment, line 5620:  Called use 'clang'
 *   phase-helpers.sh, line  247:  Called die
 * The specific snippet of code:
 *                              die "USE Flag '${u}' not in IUSE for ${CATEGORY}/${PF}"

Your insight about what is going on here would be much appreciated.

P.s.: working in a chroot-ed environment here.

build.log

emerge_info_output.txt

environment