The Hotfix in question can be installed by clicking this link [1]. It’s signed by Mozilla.
Thanks to user gpm at Hacker News, who posted this tip [2]
[1] [ignore Warning and it'll install]
https://storage.googleapis.com/moz-fx-normandy-prod-addons/extensions/hotfix-update-xpi-intermediate%40mozilla.com-1.0.2-signed.xpi
[2]
https://news.ycombinator.com/item?id=19826903