Loop Video от aaltop

If you've ever used the looping ability on Youtube but felt like it's a little
too simple in some cases, this extension was designed with that in mind.
You'd like to have the video play over and over, but want to skip those ads
at the start, middle, and end of the video. Loop Video
allows creating sections for the video: You can start a section just
after the first ad ends and end the section at the start of the midroll ad,
creating another section starting after the midroll and ending just
before the ad at the end. Now, you can start the looping, and only the sections
will play such that the ads are skipped.

Key features of Loop Video:

• Designed to work with any video, so should be usable on any site that uses the HTML video element for its video capabilities.
• Loop data can be saved per-URL so loops can be created for multiple
  pages and loaded back between browser sessions. Private session data is always
  deleted. Data can also be saved to and loaded from file, allowing making backups
  or sharing loops etc.
• Created loops can be browsed. Loops can be given titles, descriptions,
  and tags, and the list of created loops can be filtered based on these
  details. The list has links associated with each loop, allowing for
  quick access to the page the loop was created for.

See more details in the documentation.