The lags with YouTube might be caused by using ad-blockers in Firefox to discourage using such extensions where a while back YT would display a notification that an ad-blocker was detected. See this reply:

• /questions/1436659#answer-1630647 Youtube very laggy recently?

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Does it still happen with very low video quality like 360p resolution?

Thanks for the quick replies, all. As mentioned in my original post, I also suspected the adblockers were the culprit, so I have already tested in Troubleshoot mode and the problem persists with choppy / stuttering video while the audio places seamlessly. I have never changed the theme of Firefox. The issue does still happen with 360p video streaming.

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile. Make sure you stay signed out of your sync account when testing.

I refreshed my profile and with 1080p streaming the stuttering, pausing video while audio continues still happens regularly. After refreshing, it might be slightly less frequent, and it's not very noticeable in 360p, but my computer fan is still screaming even when I try watching 360p video in Firefox.. Activity monitor showing WindowServer, Firefox, and FirefoxCp isolated Web Content hovering around 10-15% CPU each at 360p.. My computer is a little old, but come on. I cranked it up to 1080p and saw FirefoxCP RDD Process jump up from 4-7% at 360p and start hovering around 40-50% CPU while the other three processes moved up a little to 12-18%.

In between testing these things, I've been using Chrome with uBlock origin to watch youtube on the machine with no issues at all.

Do your drivers need manually updated? Apple does not like to keep any former system running long. I did not see in previous posts; have you tried safe-mode on Firefox? Today's browsers are nothing compared to our Firefox. Java6< (Oracle) just loves to crash all users and all programs, no matter what. Do you get the picture? If not third party can help you install the .app's for the 1080p, then I do not know what to tell you.

Any difference with the h264ify extension installed?

Try changing layers.acceleration.force-enabled and media.hardware-video-decoding.force-enabled to true in about:config then restart the browser.

Go to about:support in the address bar and share a screenshot of the "Codec Support Information" section.

Tried the following 1. changing layers.acceleration.force-enabled and media.hardware-video-decoding.force-enabled to true - this alone not helping 2. change those settings back and install h264ify - this alone not helping 3. with h264ify installed, change the settings to true - Unfortunately, this isn't helping either.

Thanks for the suggestions. Let me know if you have any other ideas. I'd be glad to continue trying things out to get YouTube in Firefox to work properly on macOS 11.6.8.

I also decided to try streaming a couple of videos on Vimeo in Firefox, and those seem to work fine, so still not sure what's going on.