Thanks for the review, and sorry YouTube stopped loading for you.

I tested this on a real Firefox setup and found the cause of the endless loading: running two ad blockers at once. With Ghostery and uBlock Origin both on, 7 of 8 test videos never loaded, whether this add-on was installed or not. With Ghostery switched off, all 8 played in about 4 seconds.

If you have a second ad blocker (Ghostery, AdBlock Plus, AdGuard or similar), keep uBlock Origin and turn the other one off, or pause it on youtube.com. We've just deployed a new version in 1.6.1 which now shows an orange "!" on the toolbar icon when a video is stuck loading, with an explanation in the popup.

If videos still won't load after that, please tell me which ad blockers you use and your Firefox version, and I'll look into it.