UPDATED REVIEW:
Just noticed the warning that I need to let the video finish (weird that the addon cannot do it itself). Now seems to work but still it would be nice if I would not have to do math to get the seconds and input a time like a regular human being in hh:mm:ss format. None the less I updated my review and my rating to reflect the fact that now it works for me as it says in the box. Add a better input and you have 5 stars from me :)

OLD REVIEW:
trying to make this work, and even through I am a coder and almost smart enough, cannot get it to work on firefox (latest version 70.0.1 (64-bit) running on Win 10) , seems to ignore my inputs in seconds for start and end and just jumps to random locations of the video. A bug ? a feature ? A disturbance in the force ? Only the God of Coding knows, or maybe the developer too ?

I think you're the same creator for the repeat button on the old Youtube version, you could chose the time or loop the whole thing with the press of a button? It was great.

The problem right now is that your module affects every active Youtube tab, so brace yourself if you want to repeat something, EVERYTHING will be repeat at once.

I don't know how much restrictions you have right now to use an integrated button in the page, but it would be the better way to go. Plus I need to do a little math to find the time, no biggie, just not ideal.

I'd be happy to change my review once you have an integrated, tab specific, button.