I know a handful of EM users (me included) like to create and use custom themes for the site, and the most popular Firefox/Chrome add-on is Stylish.
The Stylish add-on has been (for at least 18 months now) stealing your internet history in the background. Here's a recent writeup: https://robertheaton.com/2018/07/02/stylish-browser-extension-steals-your-internet-history/
The open-sourced alternative Stylus is available and is an improvement over Stylish in every way. It took me 10s to switch over and export/import my themes.
EDIT: Stylish has been removed from the Firefox and Chrome stores.