Nice addon but it's stopped working. As of 2021-04-02, it has this problem:

When the first bookmark is made, it correctly adds the timestamp to end of the URL like so:

...&feature=youtu.be&t=21

If the video is then started with that bookmark, and the bookmark is later updated then *both* timestamps are included in the URL like so:

...&t=21s&feature=youtu.be&t=35

If the video is played from that URL, then it plays from the first timestamp, effectively meaning that the bookmark timestamps can no longer be updated with this addon.

Just added the add on to merely bookmark my video and my place in it and it seems to work. I think u need to restart Firefox. The folder appeared in the far left of my bookmark toolbar. Thanks for the extension.