I think it’s specific to Ubuntu - https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1780472
I can’t find a b5 build either so I’ve had to leave the beta channel by downgrading all the way to 61.
sudo apt install firefox=61.0+build3-0ubuntu0.18.04.1
Sometimes downgrading whole versions can cause havoc with user profile data; I’ve not noticed any side-effects like that this time but I am unhappy at such a critical part of the browser being released with no QA testing for anything other than a fully automated nightly channel. Let’s hope they fix it with a new build soon and learn some lessons from the mess.