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.