Not sure when the more universal fix will show up but there is a way to get things back to working if it is still not fixed by the time you read this.
You can check to see whether you have "studies" enabled by heading to Options > Privacy and looking for the Allow Firefox to install and run studies option. Just check the box if it is not already. Soon you will see this show up in the about:studies
This is a hotfix that updates an intermediate certificate used for signing add-ons. It is one of the mechanisms used to fix bug 1548973."
Later after they get a real fix working you can disable this again if that is what you want.
I have done this some hours ago and it seems to all be working again just like before this happened.