There is a new pull request by Duncaen against master on the void-packages repository

https://github.com/Duncaen/void-packages firefox-81
https://github.com/void-linux/void-packages/pull/25049

firefox: update to 81.0.
x86_64-glibc works, currently building for aarch64 and then building x86_64-musl which I can test.

A patch file from https://github.com/void-linux/void-packages/pull/25049.patch is attached