"if everyone goes off and gets vaccinated, would you?"

Apple constraints their users for certain reasons which they believe are good for the user (with some negative side effects that Apple also likes). Mozilla likely has a similar motive; doing good for their users.

It doesn't mean every user will be happy with that, the average user will however be better off. The non-average user can then install the developer or nightly edition of firefox or even compile it themselves too get unsigned addongs if they so choose. Choosing the branded release branch of firefox means that Mozilla wants to keep you safe to some extend. This means not allowing third parties to install arbitrary addons into your browser, for example. Users rarely know what they want or if they do, they go about it in destructive ways.

Install the developer edition and you get your "I want to be able to load any extension".