Firefox

User Styles

Enable user styles by going to about:config and finding the toolkit.legacyUserProfileCustomizations.stylesheets setting, and toggling it to true.

Open up the Troubleshooting page to find your user profile location (hit ALT > Help > More Troubleshooting Information, or Hamburger Menu > Help > More troubleshooting information).

Find the Profile Folder setting and click Open Folder

Create a chrome sub folder, and in here you can create your userContent.css.

Restart Firefox

Recommended settings

browser.tabs.closeTabByDblclick to true browser.tabs.insertAfterCurrent to true

Advanced

about:performance about:memory

about:processes Is a much better task manager

toolkit.aboutPerformance.showInternals