There are (sadly) plenty of people out there who will install a browser extension & tweak an about:config setting if you set up the right incentives to get them to look past the warnings.
The price of protecting these people from those that would take advantage of them is developers having to download a different version of the browser in order to develop their extensions.
They claim to be worried about malware installing their own extensions, so they lock everything down. As if that same malware wouldn't just patch the firefox binary to allow loading unsigned extensions.
It's ridiculous and obviously meant as a first step towards a walled garden. It's very unfortunate that firefox followed google in this.
Check out Firefox for Android. It has a Walled Garden of extensions. There is a list of about 20 whitelisted extensions. All others are (falsely) claimed to be "incompatible".
I absolutely get this argument and this is also what I got from the Mozilla docs. For me the decision for user safety moved too far away from user freedom. I get that this is a tradeoff, I am not happy with the outcome personally. I have no perfect solution to this problem either, I just find the current state runs counter to what I personally would expect.
I see a sliding scale from Apple walled garden to Windows XP wild west. I think I am more on the XP side than the Apple side, so my view likely clashes with what Mozilla has in mind.
So what stops these people from downloading a developer version of the browser themselves?
You cannot protect people from themselves. The only cure for idiocy is death after all.
They don't do it because it doesn't stop those plenty of people from doing what they want to do. This misguided decision from Mozilla only serves to drive developers and tinkerers away from their browser. (No, I will not download the nightly/beta browser just to run my own addons)
Why be vague? Point out some people who were abusing the previous system who couldn't abuse it now.