[gb2world]

I think it is a little easier than that to get directly to the correct css - depends on how you have firebug set up.

Select Inspect in the firebug console menu.
Run your mouse over the part of the page that you need information - it should place a border around the html element. (It could be a div, span, a, td, etc.)
Right-click it and the html will be highlighted in the html pane.
Now, in the html pane, Left click the html and the style pane takes you directly to the css controlling the html element. It tells you the name of the css file and the line number
If it is in-line css, you will not see a css file name and line number in the css pane. Instead - you will see "element.style" and you will see the css in the html pane. This means the css is directly assigned in the html (in the template)
For some things that you can update using the WYSIWYG - you could have inline css style that is not in the template - it is in the database. You update those in your administration with the WYSIWYG, with a csv update, or however you update the database