It should also be pointed out that the Firefox devs spent years and countless dev hours trying to keep the old extension system and solve the problems wrt. multi-process, security, performance, and compatibility. They removed the extension system only after they tried everything else, and mostly failed.

They also spent tons of effort explaining the background of these choices and why they felt they had no choice and this was the only path forward. It's disappointing people are still coming up with this "oh, why don't they just [..]?!" type stuff.