Download YouTube videos. Video formats are listed at the top of the watch page. Video links are tagged so that they can be downloaded easily.
I finally have a chance to look at Greasemonkey 4.
It turns out that GM developers provide code sample how to make scripts compatible with GM 4. (I must have missed it in my haste.)
So, v2 now works in Greasemonkey 4. I had to modify the script a bit to make it work 100% on GM 4, though. There is one functionality that does not work on GM 4. Buggy, perhaps?
[Fixed] Not working with Greasemonkey 4 & Firefox 57
I did not update to Firefox 57 in advance, so now that it is available to the public, I found that YouTube Links does not work with it and Greasemonkey 4.
Some of the GM API has changed and it might take sometime to make it work again. An issue is if the changes will be compatible with other scripting engines, such as TamperMonkey.
In the mean time, you can use TamperMonkey. It sort of works. The top bar doesn't show up sometimes due to timing issues.