Building firefox on amd64 musl based system with rust-1.56.1::musl yields the error:
firefox_build/dist/system_wrappers/new:3:15: fatal error: 'new' file not found
Created attachment 758731 [details]
Created attachment 758732 [details]
Doing some more digging, the problem in the failing clang command seems to be the "--target=x86_64-unknown-linux-musl", which should be "--target=x86_64-gentoo-linux-musl".
clang++ --target=x86_64-unknown-linux-musl test.cpp for test.cpp containing only "#include_next <new>" reproduces the error (for both clang-13.0.0 and clang-12.0.1).
enable default_libcxx will workaround your issue at the moment, we have diagnosed this several times on #gentoo-hardened.
Enabling default-libcxx for sys-devel/clang allowed firefox to compile, thanks.