Earlier quoted context omitted.
Just export the key from a current version and import it into the old version. Worked for me.
How do you do this?
Preferences -> Privacy & Security -> Security -> Certificates -> View Certificates
Now find:
Mozilla Corporation
signingca1.addons.mozilla.org
Select signingca1.addons.mozilla.org and then choose export. This is what you would import into the older version of Firefox.