The first CFLAGS (or CXXFLAGS) line below appears to be taken from the
environment setting, and the second is from the "<packagename> was built
with the following" section of the emerge --info output.

Squid looks as it should (the -flto is added to that package only) but for
firefox it appears that -O2 has been stripped, and not replaced with -O3,
which IIRC is the default for that package.

# grep ^CFL /etc/make.conf
CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe"

# emerge --info squid | grep ^CFL
CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe"
CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe -flto"

# emerge --info firefox | grep ^CFL
CFLAGS="-march=amdfam10 -mcx16 -msahf -mpopcnt -mabm -O2 -pipe"
CFLAGS="-march=amdfam10 -pipe -mno-avx"

Is anyone getting O2 or O3 in their emerge --info firefox output?