Re: Toy extensions won't mess with their revenue stream.
I have a lot to complain about Firefox choices myself, but why they chose this default is so easy to explain I wonder why I'm having to.
Computer-illiterate people never look into Settings (even if they know such a feature exists). For their own security, updates need to be as automatic as possible, constantly asking would only result in constant uninformed refusals ("WTF, leave me alone already!"). No, this is not an assumption, this is an observation.
Computer literate people on the other hand, the ones needing/wanting more control, have usually heard about those legendary "Settings" menus, and will thus stumble upon the possibility to remove the training wheels. It's there, for them.
(Didn't downvote you though.)