The attack surface isn't increased. If you can exploit the vulnerability using Shumway, you could also exploit the vulnerability using vanilla JS — because Shum

The attack surface isn't increased. If you can exploit the vulnerability using Shumway, you could also exploit the vulnerability using vanilla JS — because Shumway is just Javascript. Shumway doesn't allow new exploits that are impossible without Shumway. Shumway decreases the attack surface of Firefox for general-purpose browsing: most people would install Flash otherwise, and this way that (infamously broad) attack vector is removed.