Just opening a new tab, when you have a lot of tabs open, is slow on Firefox.
Chrome is also much better for web video. Sites like Youtube and twitch.tv perform better on Chrome. I don't have any benchmarks on hand, but that's how I feel after switch between two browsers for a while. I think I'm sticking to Chromium.
YouTube feels very fast for me in Firefox. The v0 polyfill is not slow, you shouldn't notice any slowdown from it unless your hardware is really really bad.
Chrome is also much better for web video. Sites like Youtube and twitch.tv perform better on Chrome. I don't have any benchmarks on hand, but that's how I feel after switch between two browsers for a while. I think I'm sticking to Chromium.