I don't know what this has to do with web 2.0, but CSS buttons are fairly straightforward to make. (That's their advantage over image buttons.) Just design the style for a single button and apply to all buttons of that class. If you want to use the same style of buttons across multiple sites, just copy and paste the code and change the colors. Unless you have a hundred different button styles, it shouldn't take you more than a minute or two.
No script is going to be able to automatically determine what style or color of buttons best suits your layout. So you'll still need to pick the colors yourself. I'm not sure how a script will speed that up.
And why are you designing buttons in Firefox anyway? It's a lot easier to pick out colors (and button style) in a real graphics program while you're designing the layout and just copy and paste the color code into the stylesheet.