You aren't able to add exceptions using the Firefox preferences page. You can only remove or modify the autoplay level from there.

To add a website to the exception list, you need to do that from the Site Information drop-down panel. See the Site settings section of Allow or block media autoplay in Firefox.

If you don't see that permission listed in that panel when you are on the website you are trying to add to the exception list, it's probably because the website hasn't tried to play any content yet. That permission will only appear when the website tries to play content.

In that case, you can still modify the permission for that website using the Permissions tab from the Firefox Page Info window.

Hope this helps.

Thank you very much Wesley Why on earth did Firefox remove the ability to directly set exceptions?? (Rhetorical) - ASOLUTELY CRAZY!!!!!

Wesley Branton said

You aren't able to add exceptions using the Firefox preferences page. You can only remove or modify the autoplay level from there. To add a website to the exception list, you need to do that from the Site Information drop-down panel. See the Site settings section of Allow or block media autoplay in Firefox. If you don't see that permission listed in that panel when you are on the website you are trying to add to the exception list, it's probably because the website hasn't tried to play any content yet. That permission will only appear when the website tries to play content. In that case, you can still modify the permission for that website using the Permissions tab from the Firefox Page Info window. Hope this helps.

This solution unblocks ALL web pages, I want to add an exception to only 1 or 2 websites. Currently having an issue not allowing me to open PDF files