After a long day of anger and speculation, Google has finally commented on the "artificial wait" that some Firefox users are encountering on YouTube. This 5-second delay, which is plainly visible in YouTube's code, is made to punish those who use ad blockers. It affects all browsers, not just Firefox.
YouTube Adds 5-Second Delay to Punish Ad Blockers in All Browsers
The 5-second delay is not exclusive to Firefox users, says Google.