The issue is that most users don't understand software on a deep level, and just click "yes" on dialog boxes, etc.
It does make sense to keep the defaults where it prevents most users from harm.
And by experience supporting users, this is not how bad extensions get installed on the system: they're pulled in by malware which gets installed by other means.
This is only going to irate legitimate extension developers, which already have to wait weeks for AMO to review even the most basic change. I've been distributing extensions separately precisely for this reason.
2) Politics(especially taxes & wars)
Specialisation always was this species strong point. Acceptance that the user might have his strong-point elsewhere and is so nice as to not harass you with his worldview. Imagine if you went into your local bakery, and there behind the counter stands a guy all in white:
"Good morning. Try our donuts today. You could make donuts too. Its easy. Come on ill show you. And then you will be self reliant when it comes to donuts. There are thousands of great recipes online - okay, some are broken, but you dont get to become a expert in donut making - without giving a little bit back..
Sir, Sir - you forgot your Donuts. Maybe he is diabetic and forgot - or evil cooperate donut buyer - or the one dough ring to bind them all is too much of a power.."
With great specialisation comes great loss off understanding on other parts of your life.
Literacy took much longer, but the benefits are clear today.
Wait, did we have an extension-caused apocalypse recently leading to this requirement ? Erm, I guess not. So why do we exactly need it ?
Firefoxs plugin development is already a pain in the ass. They should focus on making it simpler, not giving more reasons to fork it.