Firefox is changing this because malware injects itself into Firefox using this mechanism. Power users who really want to install unsigned addons can switch to the Developer Edition.
Now we seem to be nerfing the beast because someone might do stupid things.
I understand why Mozilla and Google have done similar things to restrict extension installation, but can't help but think there has to be a better way for users who know what they're doing.
Browsers are putting a lot of effort into preventing this, they wouldn't do it if they didn't have clear information showing the problem was serious.