For sandboxing webapps you will need to install these optional dependencies:
wasi-libc
lld
wasi-compiler-rt
wasi-libc++
See https://hacks.mozilla.org/2020/02/securing-firefox-with-webassembly/
to learn why you might want to enable this feature. A different approach,
https://wiki.gentoo.org/wiki/Simple_sandbox , can be implemented without
any additional dependencies.
If you compile firefox frequently, consider installing ccache and sccache,
so that later updates can use the artifacts left over from previous builds.
John McQuah
d052e56fde