Caused all Youtube videos to show error Rated 1 out of 5 stars
Installed it on a fresh install of Firefox 17.
99/100 Youtube videos did not work at all, showing the "An error occurred. Please try again later" message. Did not know what caused this issue.
Finally narrowed it down to 2-3 addons and disabled each one, one by one. Disabling this one did the trick.