Was running very well 5 stars for that, then downloaded Firefox 69, now it doesn't work on any YT Videos.
The << and >> functions work, it reverts it back or speeds it up by 10 secs at a time, so that function works.
But the - and + function to speed it or slow it down by fractions of a second no longer works.
Maybe it can be fixed, nice app that was working and worked well, but not now, just can't get it to work even after deleting it and reloading it.
Such a shame, it might work for other uses but no longer works for me.

**Stop press** **Stop press**. Just found the answer to the problem. It seems after downloading a Firefox add on called, "Audio Equalizer and Amplifier" and actively enabling it, (Ticking the enable box), about same time as the Firefox 69 update, this particular Video Speed Controller app won't work with that other audio app.
The other one enhances the audio nicely, and we all know what this app does.
So I'll have to chose when it is just audio commentary, speed it up, when it is music leave the speed alone. Just unticking the enable box in the "Audio Equalizer and Amplifier" app will fix it.

Back up to a 5 star rating then!, maybe not the fault of the developer, just conflicting programs that don't work well together. It may be Worth downloading the two by the developer just to see if this video speed app can be made with further code development to run no matter what other apps are installed in Firefox.
Of cause we should understand a developer can hardly check it against every other Firefox app for compatibility it would take years and years of testing due to the vast amount of apps on the Firefox add on site.
But possibly some helpful feedback has been given here none the less. Still a decent enough app to use, it works better than the built in YT one that is only in 0.25 secs not the 0.1 sec increments of the Video Speed Controller app, which is far more precise & handy.