Custom per-site stylesheet extension for Firefox
I believe you're looking for Stylish. (official addon page)
Stylish lets you easily manage user styles. Add, delete, enable, disable, and organize with a few clicks of a mouse, no code to edit, no obscure configuration to find. Stylish's companion website, userstyles.org, hosts tens of thousands of user styles made by other Stylish users that you can try.
(Despite the above, it's just as easy to use your own stylesheets for any website).