How to permanently add CSS to some pages using Firefox?

There's a great extension called Stylish that does exactly what you're looking for. It allows you to add custom CSS to the pages you visit.

https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/

You can add custom CSS rules to a chrome/userContent.css file in your Firefox profile folder.

• Open your profile folder
• Create the "chrome" directory if it doesn't exist
• Create the "chrome/userContent.css" file

Example:

/* For any page */
body { margin: 10px; border: 1px solid red !important; }

/* Or only for google.com */
@-moz-document domain(google.com) {
    body { margin: 10px; border: 1px solid red !important; }
}