On Linux, simply copy the .css file into ~/.mozilla/firefox/XXX/chrome/ where XXX is your profile folder.
On Windows, the path can be found following this: https://superuser.com/questions/318912/how-can-i-override-the-css-of-a-site-in-firefox-with-usercontent-css
If chrome directory does not exist, create it.
Usually, your profile folder should be the biggest in size.
Finally, visit about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true