Okay I made a post about a week ago but it was a bit convoluted and things have changed so sorry for the repeat.
I'm using Waterfox running FF55 using the latest official Stylish Add-on on a fresh profile for testing and using no other add-ons.
When I go to install a style from userstyles.org and I click the button nothing happens. The Popup that usually pops up is not there.
On my other FF profile where the problem was noticed I have multiple styles that are currently working. The edit style button seems to not be functioning either. On that profile I also use Tampermonkey for some styles and it is working fine.