In case you're in need of a cross-compiling method for Firefox on gentoo i686 musl, once I've finished building/testing the result, I'll be uploading some experimental build environment configuration files/patches for gentoo x86_64 musl to GitHub, which I use to cross-compile from glibc to musl (why I began with a glibc chroot is a long story) and can easily be ported to other architectures (x86_64 glibc => aarch64 musl, for example). I used to have it working with older software versions and am just updating the work to current versions.
Another thing: some work on cross-compiling from x86_64 musl to other architectures, if that's more what you would prefer, has also been done by gentoo devs ( hint: https://github.com/smaeul/portage-overlay/tree/master/dev-lang/rust ), yet isn't in their musl or main ebuild repos yet. My recommendation would be to ask in #gentoo-hardened on Freenode IRC if you can, and see what your best approach would be after discussion and code review.
If you can't go on IRC, I'll let you know when I'm finished with testing here.
Regards,
Alexis