Spidermonkey is required for essential packages like polkit or Gnome/Cinnamon. However the compilation of spidermonkey has very high requirements on the system, which have even more increased with the version bump from spidermonkey 68 to 78: At least 6400MB of hard disk for spidermonkey 78 and very high ram usage due to rust. Therefore: Please consider providing a binary package, otherwise the usage of Gentoo on older systems with limited hardware resources will be unfeasible.
Is is not a official solution, but spidermonkey can be replaced by duktape. It has a much lower compile requirements. See bug #734326.
I may be wrong, but unlike firefox-bin I don't think mozilla distributes a usable pre-built libmozjs. There are very few cases where *-bin packages are built by Gentoo at the moment and it's upstream's binaries.
