My experience is quite good, but I met some problems following these steps:
1. install firefox without installing any other addons.(in fact, I disabled them)
2. install this and Stylus, then install a theme from userstyles.org.
3. open the website you styled, then you'll find it loaded twice instead of once. but if you uninstall this addon, you'll find the site load only once.
May I know it's a bug or feature?
from Window 10 64bit firefox 61.0.1 release