The best part about FireBug is actually JavaScript debugging and the JavaScript console. Writing a web-app would be nearly impossible without this.
For simple CSS you probably want to be changing it in a seperate text editor that auto-refreshes your web-browser so you have the benefit of being able to save.
The CSS-changing feature you mentioned is of questionable use while the JavaScript features are essential.
Firebug is great for CSS because it allows you to find the specific CSS rules for a selected element, along with the line number it appears on in the CSS file(s), helping you find it in the separate text editor in the first place. Also you can try out new rules very quickly and get instant feedback without switching programs and setting up an auto-refresh.
I switched to Webkit's JS debugging tool not long ago, it has much better warnings than Firebug.
For simple CSS you probably want to be changing it in a seperate text editor that auto-refreshes your web-browser so you have the benefit of being able to save.
The CSS-changing feature you mentioned is of questionable use while the JavaScript features are essential.