It looks like also this was against adblocker so, again, not specifically Firefox. Quote from the article itself:

The issue was initially reported as targeting Firefox users, but users online have said they’re seeing the delay in Chrome and Edge, too. Reddit and Hacker News users who’ve examined the code that appears to be causing the delay have said they see no indication that YouTube checks what kind of browser is in use. Mozilla’s senior brand manager Damiano DeMonte wrote in an email to The Verge that “there’s no evidence that this is a Firefox-specific issue.