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.