Both. I use YT on Firefox constantly, and I just explicitly tried again with a swapped user agent, and there's no issues at all, works perfectly as expected. I saw from your other reply that you use a fairly involved and heavily modifying expansion, not just a user agent switcher.

If you try to "harden" your FF, always keep in mind that a large portion of that means absolutely breaking things left and right and center. It might work, but always expect it will not. Because it's just not something anybody would ever test for when creating web pages. So you're running essentially unknown scenarios. It might be interesting input to the extension-author that this breaks, though. It might be something they think they got working. Of course, it could also be that it's "Yeah that happens, it's intentional". But might as well report it to them.