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 in
about: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.
userContent.cssabove in the
chromefolder you just created.
about:configin Firefox then set
Quit and reopen Firefox, just for luck.