Custom CSS for Firefox

A certain popular photo sharing site recently redesigned their site. I made some CSS to "fix" the issues, now I am figuring out how to get the CSS to the page. These links might help me:

http://www-archive.mozilla.org/unix/customizing.html

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles