Watching video content in Firefox causes UI sluggishness

Hello,

I'm running a "vanilla" install I think? (using Hackintosher guides).

Hardware as reported by 'About this Mac'

• iMac (27-inch, Late 2013)
• 4.08 GHz Intel Core i7
• 16 GB 2448 MHz DDR3
• Radeon RX 580 8192 MB

When watching video content in Firefox, such as Twitch streams or YouTube the rest of the OS interface will feel sluggish. For example scrolling in another Browser window will lag behind my mouse cursor dragging the scrollbar by a significant amount, and just general feeling of sluggish behavior in other apps.

If I alter the size of the video content by shrinking the browser window this will improve the sluggish feeling of the OS.

In Windows or Linux this wasn't an issue.

Any help or suggestions would be greatly appreciated.