Sorry, I thought I had ticked the box to include troubleshooting info, but maybe not. Here's a gist with that data: https://gist.github.com/jmitash/c1a310758c227bc53693e14dc68fb6cd

And the part to answer your question specifically:

"name": "Firefox", "osVersion": "Windows_NT 10.0 19043", "version": "99.0b8",

Before I removed the old video card, I downloaded and ran Display Driver Uninstaller (DDU). Not sure if you're familiar with it, but it seems to have garnered a reputation for being good at completely removing display drivers, be it from NVIDIA, AMD, or Intel.

The first thing I noticed after logging back in after installing the card was only one of my two monitors was recognized. I naturally assumed this was because all drivers had been removed, so I went to AMD's website and downloaded their driver installation tool to install the latest drivers, after which both displays were working.

Then I started noticing the video stuttering on YouTube, tried some basic troubleshooting, and made this post.

Later, I realized this issue wasn't specific to Firefox- I could download videos and experience the same problem using VLC. So I figured you're right, this is probably a driver problem.

I tried running DDU and reinstalling drivers a few times with no luck. I downloaded DXVA Checker and watching the video decode usage while playing, and observed a couple things:

1. Video decode usage was frequently at or near 100% while playing 4k/60fps videos
2. The task manager "GPU Utilization" was at basically zero, which led me to the conclusion that the Task Manager "Performance" tab is basically useless/misleading in this situation

I started to think that maybe DDU wasn't actually removing all the drivers. So I nuked everything and installed a Linux distro. The distro I installed came with the open-source Mesa drivers for AMD GPUs. I was not experiencing any stutters playing 4k/60fps videos.

I didn't really plan on switching to Linux over the weekend, so I once again wiped everything and reinstalled Windows 10. This time, I had a bit of a different experience- while Windows was in the middle of its first-time setup, my second monitor came online. Once I got through the setup, I checked Device Manager and saw that Windows had automatically downloaded and installed an older driver from AMD, version 30.0.13023.4001.

I tried playing the same YouTube video I mentioned earlier, and, what do you know, no stuttering. I downloaded DXVA Checker once more just to confirm, and I could see video decode usage was now down to 50-60%. Downloaded a couple more videos to confirm outside of Firefox, and same result.

And just to absolutely put the nail in the coffin that this is a driver issue, I downloaded the latest driver from AMD's website again, and it was back to ~100% video decode usage with stuttering. Ran DDU, waited a few minutes for Windows to redownload the older driver, and no stuttering with ~50-60% video decode usage.

I tried some higher resolution videos just for fun. At 5k/60, I think I could see some stuttering, but not very substantial. Higher than that, the card just can't keep up. Which is still pretty unfortunate considering my almost 6 year old NVIDIA card was doing better with that. I'll have to live with this for now.

Thanks for the help.

Summary: Something seems to be wrong with the newer AMD drivers on Windows. Running "Display Driver Uninstaller (DDU)", then waiting a few minutes for Windows to automatically download an older version of the driver (you can see the download in the "Windows Update" section) has for the most part resolved the issue for me. For >5k/60fps videos, the video decode performance is still pretty bad.