Just drag this link to the bookmarks bar of your browser=> Test CSS
Then, when you click the “Test CSS” bookmark, you’ll see a small window where you can type CSS for testing different rules. I often copy the rule(s) from Firebug and vary them for the new style(s) I want. Then I copy that into the stylesheet(s).
Where it beats tweaking CSS in Firebug or other browsers’ developer tools is that when you refresh the page it doesn’t nuke your styles.
Kudos and gratitude to Jon Wiley of Google. I got to attend his session “Advanced CSS Techniques” at the 2007 AccessU conference. He had this bookmarklet as part of his online presentation notes. It’s finally offline; all I did was copy and past its contents here to use on other browsers.