My apologies for the trouble! Do you use the old Youtube theme by chance?

The newest update is a complete rewrite and only supports the new Youtube theme since the old one will officially be removed soon. I rewrote the addon since there were some fundamental bugs that I wanted to fix that required some more thorough changes, and supporting the old Youtube theme would have taken a considerable amount of effort during the rewrite.

If you are using the old Youtube theme then you could simply switch to the new one (https://youtube.com/new) and everything should work as usual again. If you want to stick to the old theme then you could switch back to the previous version of this extension. To do so, you can manually install it from the version history (https://addons.mozilla.org/en-US/firefox/addon/youtube-watchmarker/versions/) and disable automatic updates for the Youtube Watchmarker in the settings of Firefox (in the "Manage Your Extensions" settings, click on the three dots next to the Youtube Watchmarker and say "Manage", then select the option for "Allow automatic updates: off") such that you stay with that version.

If you are not using the old Youtube theme then ignore my suggestions and open an issue here such that we can look into this further to find the root cause: https://github.com/sniklaus/youtube-watchmarker/issues

:: Simon