When using the Dearrow Firefox extension paired with the YouTube Subscription List View Firefox extension, all Dearrow-replaced thumbnails fail to load without fallback. It was really annoying that YouTube removed the list view from the subscriptions page, so I was glad to see that that extension was made, but I don't really want to use it without support for Dearrow.
My subscriptions page with that extension enabled and Dearrow off:
...and with that extension enabled and Dearrow on:
(Mirror issue from the YSLV repo since I'm unsure on which extension's side the problem would need to be addressed)