There is an updated pull request by zdtcd against master on the void-packages repository

https://github.com/zdtcd/void-packages firefox
https://github.com/void-linux/void-packages/pull/15847

[TESTING] firefox: fix build for armv7hf-musl
Built for:
- [x] armv7hf-musl
- [ ] armv7hf
- [ ] x86_64
- [x] x86_64-musl
- [x] i686
- [ ] aarch64
- [ ] aarch64-musl

Tested with:
- [ ] x86_64
- [ ] x86_64-musl

A patch file from https://github.com/void-linux/void-packages/pull/15847.patch is attached