The system has 4GB of RAM an more can not be added.
The requirement for firefox is rust, but rust can not be compiled on this machine because it aborts while pre check.
Steps to Reproduce:
1. emerge -1 dev-lang/rust
Running pre-merge checks for dev-lang/rust-1.32.0
There is NOT at least 4 GiB RAM
"There is NOT at least 4 GiB RAM" its only a warning but not an error in case the is enough memory at least with swap enabled.
If not enough RAM available but swap exists this takes a long time to compile but no abort.
Memory on this machine:
total used free shared buff/cache available
Mem: 3.8Gi 1.9Gi 1.3Gi 8.0Mi 729Mi 1.6Gi
Swap: 6.0Gi 162Mi 5.8Gi
Set severity to critical because its no longer possible to update firefox so security holes can not be closed!
you always can use rust-bin to update firefox if it's that critical for you,
also you can use (if you know what you are doing ;-)
I_KNOW_WHAT_I_AM_DOING=true emerge -u rust
this will just print a warning instead of aborting the build.
the build still likely fail later with only 4GB of ram, unless you use zswap, zram or some other compressed memory trick.