New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/30445#issuecomment-827568328

Comment:
>  its not not present on 78.8, but videos do play.. however anoying.

Seems some of it is known: https://aur.archlinux.org/packages/firefox-esr

> Also you can low security.sandbox.content.level to 1 as a workaround, so you don't need to disable it completely.

If you have a mozilla bugzilla account, you should be able to report it.