Youtube annoyance - videos not starting exactly from beginning.

Over the past week or so, I've been finding that Youtube videos don't start from the very beginning - the very opening is missing. So as examples, you're playing a song that you know starts directly on a beat, but that first beat gets missed, or there will be an opening sentence such as "Everybody is talking about the latest... " and what you hear is more like "Vrybody" - you can clearly hear that It's slightly off. This relates to Firefox in three ways. My initial thought was that there is some conflict with using an Adblocker (uBlock in my case) as Youtube is trying to force a pre-video ad that won't play, but even with the adblocker disabled the problem persists. I then cleared my Firefox Youtube history so it loads as a completely fresh version - but the problem is still there. Finally, I tried the same video on Chrome to see if this is definitely a Youtube issue...and it plays correctly without a problem. Therefore, the problem seems confined to Firefox but I am unsure where - what setting needs to get tweaked to sort this out?