YouTube has clarified in a statement that users who use ad blockers will have a suboptimal experience regardless of their browser.
This delay isn't exclusive to Firefox, it's in any browser where a user has enabled adblockers including in Chrome. Disable the adblocker and this delay magically go away. This was done on purpose.
Thank you. I happened to be smelling something toast like at the same time I tried to read that and had to check the comments. Turns out it was a heater that hadn’t been on in a while.
It is my first, and I definitely had to read it twice to make sure that I understood. It is definitely not you, but the awkward way they worded the title.