Thanks for the review! And I'm sorry to hear the extension isn't working as expected.

Regarding your first point, disabling the View Threshold slider just locks the threshold value at 100%. This means that only videos which have been watched in their entirety will be hidden by the extension. Let me know if you have any ideas or suggestions for making the View Threshold toggle more intuitive!

Also, the Material Icons font from Google Web Fonts is used to render icons in the popup window so disabling remote fonts in Firefox will interfere with how the user interface is presented. To my best knowledge, the only solution is to re-enable remote fonts in Firefox.