This extension adds a switch to toggle auto replay of the current YouTube video.
It looks like this:
Firefox Addon: https://addons.mozilla.org/en-US/firefox/addon/autotube/
Edge(Chromium) Extension: https://microsoftedge.microsoft.com/addons/detail/infbdedbjcieffdjgbebhkdppgclppai
Chrome Extension: Download zip from releases, extract to a folder, activate Developer Mode in chrome://extensions and load the unpacked extension