Not exactly. You can install add-ons from outside of Mozilla add-ons site. The extra certificate is more of Mozilla's seal of approval. This is why quite a few

Not exactly. You can install add-ons from outside of Mozilla add-ons site. The extra certificate is more of Mozilla's seal of approval. This is why quite a few of my add-ons were not disabled - they were installed with trust from another site and this intermediate certificate was never in chain. You could even manually sign these add-ons you trust with custom imported CA key for your personal or corporare vetting.