I just installed an Add-on for FireFox called HTML 5 player.
Less Resolution but fast and smooth so far.

Link: [link to addons.mozilla.org (secure)]

I also installed The NoScript Firefox extension
GLP is remarkably faster and no lag at all.

Link: [link to noscript.net (secure)]

I am in IT and though it is too early to be able to conclude for certain, I am pretty sure this will solve our problems with flash and Firefox and YouTube.

The only downside is lower resolution with the HTML5 player. It supports 360p and 720p currently but there is a beta release that supports 4k so more than likely this will be a non-issue soon.

NoScript has really impressed me. GLP for example loads far quicker and my experience on GLP has been greatly improved.
Finally! Browsing is pleasant again. There are other very similar alternatives to NoScript but not as highly rated.

There is a slight learning curve as in you have to configure certain options on site by site basis or page by page.
Not difficult though. Operations such as posting on GLP will require allowances in the options menu at the bottom of Firefox.

No malware or adware in these so you have nothing to lose.

Post back and so we can determine if this might be a good solution for everyone who is having problems.