Personally I've been using Video Speed Controller, which seems more popular. One of the things that revolutionizes your viewing experience - learn the default keybindings (s -slower, d - faster, g - toggle fast (1.8x speed)) and never be bored again.
Always nice to know of other options. (The keyboard shortcuts for the extension I linked are + and -, with * being reset to default speed.)
Does the extension you linked do pitch correction as well? I wish I could find one that does for Firefox, too, if you happen to know of one. The only playback speed control extension I've found for FF doesn't.
Try setting the property manually using the JavaScript console and see if you get pitch correction. If not, maybe make sure you're on the latest version of Firefox?
https://chrome.google.com/webstore/detail/video-speed-contro...