Modifications to your theme's CSS made with Firebug are lost when refreshing your page. If you want your CSS changes to be a bit more permanent, for example to try them with different pages of your Moodle installation, you can use another Firefox plugin: "Stylish"(https://addons.mozilla.org/en-US/firefox/addon/2108). That way you can change your site's CSS with a simple mouse click without having to change Moodle code.
(See also "Are there tools which help me creating and editing themes?")
Created a separate page for Stylish.