https://support.mozilla.org/en-US/questions/1270922 Please credit the original owner.

Before I composed my origional post I have already:

1) tried safemode 2) restarted without the addons (all disabled) 3) refreshed firefox

did you try the methods listed above?

https://support.mozilla.org/en-US/questions/1235697

I have already tried the suggestions provided in the first link (restart firefox with addons disabled and refresh). Been there done that. It didn't solve the problem. Now you provide a second link to Thunderbird email client issue? What's that got to do with YouTube?

Normally one active or current youtube tab with a video playing in it consumes around 30% of CPU during any sustained amount of time (not counting incidental and very temporary one or two second peaks to just above 50% when refreshing the page or things like that). Now it immediately jumps to a minimum of 80%-90% as soon as I click the youtube clip-playing tab (in order to bring it upfront). It stays in that mode so long as I don't go to another tab.

Then sooner rather than later it moves up to 99% rather than down. You can't do much of anything else with your computer if one firefox tab consistently consumes nearly everything without relenting. So this is simply unacceptable.

I still want an answer

It's incredible that Firefox isn't tested on such a common website.

Found a few support threads that advised to disabling Hardware Acceleration and that fixed it for me.

How to disable it: https://support.mozilla.org/en-US/kb/forum-response-disable-hardware-acceleration

I'm gone assume the problem with the recommenced settings for HA is it told firefox to use all my cpu cores and because all my cores were being used to support my tab addiction the spikes would occur. Shout out to FredMcD for providing the solution in an older thread

Thanks for the heads up.

I'll see if that (HA) is what's causing this.