Very good, works as advertised. It's just that even with a modern 6-core 4.5Ghz CPU and 32GB of RAM, the sound quality in speed-up is really bad (much worse than Enounce Myspeed or many Chrome extensions doing video speed up).
If the author would improve the sound quality (give a selection for low-cpu-impact/bad quality and higher-cpu-demand/super-quality audio algo), then it would be perfect 5 stars.
Unfortunately this is a Firefox issue with not enough priority. We should bring more attention to this bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1344756
