I saw this myself when this was news. Created empty firefox profile, installed only userscript changer plugin.

Default user agent - rotating loading circle before video starts playing. Windows/Chrome user agent - video starts immediately.

Tried with multiple videos, changing first user agent that opened the video to make sure it’s not cached somewhere.

Didn’t bother to install Chrome for reverse test though.

Now it’s back to loading at the same speed regardless of user agent though.

Unless you consider fighting adblockers a futile stupidity, you should first apply Occam’s razor - explanation requiring least amount of assumptions is probably the correct one.

In this case spoofing user-agent string of Chrome is enough to fix all the performance issues on Firefox, meaning there is no fancy anti-adblock code or anything like that.