How do you do this?

From an updated Firefox navigate to

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.

BTW that certificate list if fairly long, has no scrollbar and no search box. That's a bad UI.

Is there a way to report this without spending hours to register at Bugzilla and file a proper bug report?

Not sure why you don't have a scrollbar. On my version of Firefox (Firefox Developer Edition 67-0b16) the certificate list does have a scroll bar and can be navigated using the mousewheel. Can't speak for stable Firefox since I don't use that version.

No, not that I am aware of. In my own experience it actually took me longer to find alternative ways of bringing awareness to a bug I was having than it did to signup to Bugzilla and report it there.