Hi there, thanks for asking about this feature. This is definitely available on Firefox but it will need to be enabled from the AdBlock Settings page. The following article should help guide you through accessing this: https://help.getadblock.com/support/solutions/articles/6000067131-can-i-use-adblock-and-still-allow-ads-on-my-favorite-youtube-channels- If you run into any problems, please reach out to us at help@getadblock.com

- AdBlock Support

That's very odd. Please reach out to us at help@getadblock.com and we'll take a look. Please be sure to include screenshots of what you are seeing in the add-ons and extensions menu.
-AdBlock Support