Apple has some insane policy where every browser on their mobile devices (iPhone and iPad) must run Safari's Webkit browser engine. So even though desktop and Android Firefox runs Quantum (including Mac OS), it's actually running Safari Webkit on iOS devices. The Safari Webkit engine doesn't have the option of running add-ons, much like your Safari browser.
Feel a little violated? I would feel that way, too. It's just one of the many fabulous ways Apple decides which angles of freedom you're allowed to have. If you don't want this, vote with your wallet on your next device. Money is the only motivating factor for Apple, it seems like.