This extension has been an improvement on the older Alternative Player for Twitch.tv for sure, but it does have one flaw the old player didn't in my experience:

Saved/pinned tabs do not load correctly upon opening the browser if the channel is not live. Because of this, I have to refresh saved tabs and then watch individually for about 3 seconds to get the player to load. I tried refreshing them all at once but it lead to the hourglass hanging on many tabs.