Yep. Nothing is faster than just having a browser window open on one half of the screen and a simple text editor in the other, and just refreshing your browser to see your CSS changes.
However StyleMaster - http://westciv.com/style_master/index.html - does do a nice live preview, as well as having some other nice editing features.
But in general - you can't beat the following 2 FF extensions: the webdeveloper toolbar, and the 'View Source Chart' CSS/HTML diagrammer. FireBug is OK too, but the webdev toolbar has most of Firebug's functions.
And I agree with you about Opera. Everytime I use it I am more and more impressed - a very elegant product, without the massive bloat and RAM hoggery you see in IE and FF.
If I didn't have to pay attention to the fact that so many web users use FF and IE, I think I would use Opera more.