It seems that in the last day or so, Mozilla forgot to renew an intermediate certificate in their FireFox browser. The result is that the browser has mass-disabled add-ons.
One temporary solution someone mentioned (which may only work on a Dev version, and/or Linux version):
Simple temporary fix is to browse to "about:config" and change the value for "xpinstall.signatures.required" to false. Well, that worked for me, anyway. When Mozilla fixes whatever the issue is, I'll set it back to true.
The 'studies based' alternate solution:
You can turn on the Studies option under Privacy & Security for a little while, then check about:studies and you should see one or two in there regarding the xpi verification/signing. Once you have those two studies, you can disable Studies again.
Mozilla has an Update Regarding Add-ons in Firefox, but does not apply to Firefox ESR (my Debian Linux install) or FireFox for Android.
Another link about resolution: Certificate issue causing add-ons to be disabled or fail to install