The UI doesn't tell you, unfortunately. You would have to verify it out of band. But the browser already forces the verification, which you can verify by noticing all your add-ons are literally disabled because the signature checking is failing on them.

AFAIK the reason the UI is scary is primarily because it is from an era when add-ons were much more dangerous, and when they were not required to be centrally signed. Neither is true anymore afaik.