Flash's security track record is still far, far behind Mozilla's. There aren't that many runtimes in the world that find themselves in a position to be constantly scrambling to patch egregious 0-day exploits, but somehow Flash managers.

Firefox is to Flash like a solid wooden door is to a screen door.

Even if it isn't bulletproof, it's still a huge improvement over the status quo.

And keep in mind this solution precisely fits your desires: "less code and business built-in to the browser". This Flash player runs directly on top of the existing sandbox and does not create yet-another-special-case-for-native-code. It introduces little to no additional attack surface area.