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

https://github.com/jnbr/void-packages firefox
https://github.com/void-linux/void-packages/pull/14646

firefox: update to 69.0.1
do not define _FILE_OFFSET_BITS anymore as it now breaks things
fixes #13671

A patch file from https://github.com/void-linux/void-packages/pull/14646.patch is attached