To install in Firefox:
-
If it doesn't already exist, create a folder called
chromein your profile folder. The easiest way to find your profile folder is inabout:profiles(there's a "Show in Finder" link for the folder on macOS). Be sure to choose the right profile is you have more than one. -
Put
userContent.cssabove in thechromefolder you just created. -
Load
about:configin Firefox then settoolkit.legacyUserProfileCustomizations.stylesheetsto true. -
Quit and reopen Firefox, just for luck.