Once firefox-52.0.1-r1 at the time of speaking is compiled, a relocation error appear: firefox: relocation error: /usr/lib64/firefox/libxul.so: symbol png_get_first_frame_is_hidden, version PNG16_0 not defined in file libpng16.so.16 with link time reference Downgrading to firefox-45.8 seems to fix the issue but is not a viable option for testing users. It is a runtime issue and not a compilation issue, compilation has been both produces with and without ccache.
Portage 2.3.5 (python 2.7.13-final-0, default/linux/amd64/13.0/desktop/plasma, gcc-4.9.4, glibc-2.24-r1, 4.9.11-gentoo x86_64) ================================================================= System uname: Linux-4.9.11-gentoo-x86_64-Intel-R-_Core-TM-2_Duo_CPU_P8600_@_2.40GHz-with-gentoo-2.3 KiB Mem: 8101912 total, 1347476 free KiB Swap: 0 total, 0 free Timestamp of repository gentoo: Sat, 25 Mar 2017 00:45:01 +0000 sh bash 4.4_p12 ld GNU ld (Gentoo 2.27 p1.0) 2.27 distcc 3.2rc1 x86_64-pc-linux-gnu [disabled] ccache version 3.3.4 [enabled] app-shells/bash: 4.4_p12::gentoo dev-java/java-config: 2.2.0-r3::gentoo dev-lang/perl: 5.24.1-r1::gentoo dev-lang/python: 2.7.13::gentoo, 3.5.3::gentoo, 3.6.0::gentoo dev-util/ccache: 3.3.4::gentoo dev-util/cmake: 3.7.2-r1::gentoo dev-util/pkgconfig: 0.29.2::gentoo sys-apps/baselayout: 2.3::gentoo sys-apps/openrc: 0.24.2::gentoo sys-apps/sandbox: 2.10-r4::gentoo sys-devel/autoconf: 2.13::gentoo, 2.69-r2::gentoo sys-devel/automake: 1.11.6-r2::gentoo, 1.13.4-r1::gentoo, 1.15-r2::gentoo sys-devel/binutils: 2.27::gentoo sys-devel/gcc: 4.9.4::gentoo, 5.4.0-r3::gentoo sys-devel/gcc-config: 1.8-r1::gentoo sys-devel/libtool: 2.4.6-r4::gentoo sys-devel/make: 4.2.1::gentoo sys-kernel/linux-headers: 4.10::gentoo (virtual/os-headers) sys-libs/glibc: 2.24-r1::gentoo Repositories: gentoo location: /usr/portage sync-type: webrsync sync-uri: rsync://rsync.gentoo.org/gentoo-portage priority: -1000 local_overlay location: /usr/local/portage masters: gentoo priority: 0 steam-overlay location: /var/lib/layman/steam-overlay masters: gentoo priority: 50 ACCEPT_KEYWORDS="amd64 ~amd64" ACCEPT_LICENSE="* -@EULA" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-O2 -pipe" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/lib64/libreoffice/program/sofficerc /usr/share/config /usr/share/gnupg/qualified.txt" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo /etc/texmf/language.dat.d /etc/texmf/language.def.d /etc/texmf/updmap.d /etc/texmf/web2c" CXXFLAGS="-O2 -pipe" DISTDIR="/usr/portage/distfiles" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs ccache config-protect-if-modified distlocks ebuild-locks fixlafiles installsources merge-sync news parallel-fetch preserve-libs protect-owned sandbox sfperms splitdebug strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync webrsync-gpg xattr" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="http://distfiles.gentoo.org" LANG="fr_FR.utf8" LDFLAGS="-Wl,-O1 -Wl,--as-needed" MAKEOPTS="-j3" PKGDIR="/usr/portage/packages" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages --exclude=/.git" PORTAGE_TMPDIR="/var/tmp" USE="X a52 aac acl acpi alsa amd64 berkdb binding bluetooth branding bzip2 cairo cdda cdr cli consolekit cracklib crypt cups cxx dbus declarative dri dts dvd dvdr egl emboss encode exif fam firefox flac fortran gdbm gif glamor gles gpm gtk iconv ipv6 jpeg kde kipi kwallet lcms ldap libnotify mad mng modules mp3 mp4 mpeg mtp multilib ncurses nls nptl ogg opengl openmp pam pango pcre pdf phonon plasma png policykit ppds pulseaudio qml qt3support qt4 qt5 readline sdl seccomp semantic-desktop session spell ssl startup-notification svg tcpd threads tiff tk truetype udev udisks unicode upower usb vorbis wayland widgets wxwidgets x264 xattr xcb xcomposite xinerama xml xscreensaver xv xvid zlib" ABI_X86="64" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" APACHE2_MODULES="authn_core authz_core socache_shmcb unixd actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias" CALLIGRA_FEATURES="kexi words flow plan sheets stage tables krita karbon braindump author" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="mmx sse sse2 mmxext" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock isync itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf skytraq superstar2 timing tsip tripmate tnt ublox ubx" INPUT_DEVICES="libinput keyboard mouse" KERNEL="linux" L10N="fr" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LINGUAS="fr" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-6" PYTHON_SINGLE_TARGET="python2_7" PYTHON_TARGETS="python3_5 python2_7" RUBY_TARGETS="ruby21" USERLAND="GNU" VIDEO_CARDS="amdgpu fbdev intel nouveau radeon radeonsi vesa dummy v4l" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account" Unset: CC, CPPFLAGS, CTARGET, CXX, EMERGE_DEFAULT_OPTS, INSTALL_MASK, LC_ALL, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS, USE_PYTHON
are you still able to duplicate this issue?
Unfortunatelly no but I think this was due to other corrupted libraries and not firefox itself, I fixed it a bit ago now
Please feel free to reopen if you can reproduce the failure or if you need help in the future.