Hi, I followed your instructions to the letter. The Greasy Fork script opened after I hit play on the video, but the Video and Audio don't seem to be downloading. (see image - now stuck like this for 10 minutes). (p.s. I have two different browsers open at the same time - the screen on the right is the Firefox browser (your instructions), and the screen on the left is the Chrome browser that I used to download TamperMonkey and Greasy Fork. Thanks for any help with this. (Windows 10 computer - should have plenty of hard disk space).
Updated: After 10 minutes, it started downloading, but the download rate says 17.43kB/s and it appears to be stuck again. Tested my internet download speed and it's currently over 200MB/s. Video is about 1% downloaded, Audio is about 15% downloaded...but not moving.
Oh great. I googled "Merging Video, please wait"...found a link to a Greasy Fork user forum in Chinese and when I clicked on translate, seems that 95% of YouTube videos using Greasy Fork get stuck at this stage. What a waste of time. Please check the details and test widely.