Okay, but Linux is an operating system (kernel), Firefox is a web browser. That's two completely different things with different expectations and threat models.
Chrome lets you do it, and with the market share that it has, it means that it's the de facto expectation that you can actually install extensions without an account. And it's not like chrome is less secure than firefox, the opposite is true actually.
if this is horrifying, perhaps you should go touch some grass man. A decent security take is not a horror show when you can get even hobby stuff of your own signed fairly trivially.
Sorry, what security is this offering? At the cost of my PII?
And Mozilla still doesn't scan or do any due diligence on whatever it is you are installing.