Nice! Like older firefox extension to do site customizations.
I think strange separate tab for configuration, maybe more easily on the page(Over)
And complex to do configuration... But cool.
And that problem:

17:10:14.762 TypeError: localStorage is null D5S6p27u5t8:11:214
https://www.youtube.com/embed/D5S6p27u5t8:11
https://www.youtube.com/embed/D5S6p27u5t8:11
moz-extension://e78eb895-53d3-4535-84ac-eaeb403d6b63/embeds.js:32
apply self-hosted:4419
applySafeWithoutClone resource://gre/modules/ExtensionCommon.jsm:539
applySafe resource://gre/modules/ExtensionCommon.jsm:526
applySafe self-hosted:1007
wrapPromise resource://gre/modules/ExtensionCommon.jsm:740