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

https://github.com/zdtcd/void-packages firefox-esr
https://github.com/void-linux/void-packages/pull/15753

[WIP] Firefox esr 68.2.0
Built for:
- [x] x86_64
- [x] x86_64-musl
- [x] aarch64
- [x] aarch64-musl
- [x] i686 (Testing without `-D_FILE_OFFSET_BITS=64`)
- [x] armv7hf
- [x] armv7hf-musl

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

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I'm trying to remove the conflict between firefox and firefox-esr

A patch file from https://github.com/void-linux/void-packages/pull/15753.patch is attached