If updating didn't work I would file a bug report with Mozilla. This is the first incident I've heard of around the cert debacle where updating didn't correct the issue so what you're experiencing could be a bug.

I use Firefox Focus most of the time instead of the full blown Firefox, and its so stripped down that it doesn't support addons (and so wasn't affected by the cert issue). In fact it doesn't even really support tabs. It does help me not have a trillion tabs open and focus on one thing at a time. Depending on your browser needs it's a good, albeit totally different, experience.