Create fresh prefix environment and emerge firefox. It will not compile, but should.
Created attachment 644816 [details] build.log
from my experience with cross compiling firefox: the build system is very delicate when it picks up its clang for compiling stylo (style is part of it I believe) if you ever find a solution for this I want to know about it :-S
Can you please retry with firefox-78.3.1+ which migrated to EAPI7?